In a case before Supreme Court of West Australia when it was construing the provisions of Racial Discrimination Act 1975 (Cth) and Equal Opportunity Act 1984 (SA) it had observed that,’ As mentioned above, the Racial Discrimination Act, by virtue of s 6A, does not now purport to cover the field of racial discrimination. In those circumstances the mere co-existence of the two differing laws does therefore not render them inconsistent. The position was explained succinctly by Dixon J in Ex Parte McLean:
‘When the Parliament of the Commonwealth and the Parliament of a State each legislate upon the same subject and prescribe what the rule of conduct shall be, they make laws which are inconsistent, notwithstanding that the rule of conduct is identical which each prescribes, and sec. 109 applies. That this is so is settled, at least when the sanctions they impose are diverse. But the reason is that, by prescribing the rule to be observed, the Federal statute shows an intention to cover the subject matter and provide what the law upon it shall be. If it appeared that the Federal law was intended to be supplementary to or cumulative upon State law, then no inconsistency would be exhibited in imposing the same duties or in inflicting different penalties. The inconsistency does not lie in the mere coexistence of two laws which are susceptible of simultaneous obedience. It depends upon the intention of the paramount Legislature to express by its enactment, completely, exhaustively, or exclusively, what shall be the law governing the particular conduct or matter to which its attention is directed. [Emphasis added].It was also stated that,’ As the Commonwealth law does not cover the field, and as there is no direct inconsistency between the Racial Discrimination Act and the Equal Opportunity Act, Part 4 of the Equal Opportunity Act is capable of operating concurrently with the Racial Discrimination Act because it does not interfere with the Racial Discrimination Act. The second limb of the test contained in s 6A of the Racial Discrimination Act is therefore met. The remaining provisions of s 6A can therefore operate to their full effect, thus enabling the relevant provisions of the Equal Opportunity Act to take effect when invoked by the respondent.’
Generally, a State law will be held to be inconsistent with a Commonwealth law for the purposes of section 109 on one of three bases – that the Commonwealth law evinces an intention to “cover the field” on a particular topic to the exclusion of any State legislation, or that there is some direct inconsistency between the relevant provisions in the sense that it is impossible to obey both laws, or that a State law prohibits or permits something which is expressly or impliedly permitted or prohibited by a federal law.
The first question in a consideration of any possible inconsistency is one of statutory construction. So far as different constructions appear to be available, a construction that would allow a state statute to operate consistently with a federal statute should be preferred. In this regard, it should be noted that section 13 of the Acts Interpretation Act 1915 (SA) relevantly provides:
‘A statutory ... instrument ... will be read and construed ... so that, where a provision of the instrument, or the application of a provision of the instrument to any person or circumstances, is in excess of ... power, the remainder of the instrument, or the application of the provision to other persons and circumstances, is not affected.’
If a State law is not properly described as both “capable of operating concurrently” with the Racial Discrimination Act, and a law which “furthers the objects of the Convention”, it will not meet the threshold test and will be inoperative. That will be so because, except where the threshold test is met, the Racial Discrimination Act is intended to cover the field of the elimination of racial discrimination. As Gibbs CJ observed in University of Wollongong v Metwally:
...the Commonwealth Act, as amended, shows that the intention not to exclude or limit the operation of a law of a State exists only in relation to a State law “that furthers the objects of the Convention and is capable of operating concurrently with this Act.
When a law of a State is inconsistent with a law of the Commonwealth and becomes, to the extent of the inconsistency, invalid, the invalidity is brought about by s.109 [COMMONWEALTH OF AUSTRALIA CONSTITUTION ACT - SECT 109 Inconsistency of laws ‘When a law of a State is inconsistent with a law of the Commonwealth, the latter shall prevail, and the former shall, to the extent of the inconsistency, be invalid.’] of the Constitution and not directly by the law of the Commonwealth. The Commonwealth Parliament cannot enact a law which would affect the operation of s.109, either by declaring that a State law, although not inconsistent with any Commonwealth law, shall be invalid, or that a State law which is inconsistent with a Commonwealth law shall be valid. If there were a direct conflict between a Commonwealth law and a State law as, for example, where one law forbids what the other commands, or one takes away a right which the other confers, an assertion in the Commonwealth law that it was not intended to be inconsistent with the State law would be meaningless and ineffective. However, when there is no direct inconsistency between the two laws, the question is whether the State law is inconsistent with the Commonwealth law because the latter intends to cover the subject matter with which the State law deals, and an indication in the Commonwealth law of the intention of the Parliament in that regard would be material and in most cases decisive..
It is perhaps possible to imagine a case in which a Commonwealth Act did in truth fully cover the whole field with which it dealt, notwithstanding that it said that it was not intended to do so, but such a case may be left for consideration until it arises.
It is said in Butler v. Attorney-General (Vict.)  that "invalid" in s.109 of the Constitution means, not void, but "inoperative", so that if a State law, which was inconsistent with a law of the Commonwealth, was not repealed by the State legislature and remained on the statute book, the expiration or repeal of the Commonwealth law would have the result that the State law would come into force; in those circumstances the State law would have remained in abeyance during the time when the inconsistency existed and, when the inconsistency no longer existed, would cease to be inoperative: see at pp.274, 278, 282-283 and 286. It follows that if a Commonwealth statute which, on its proper construction, had revealed an intention to cover exclusively and exhaustively the subject matter with which it dealt, so that in consequence a State statute dealing with the same subject matter was rendered inoperative, were subsequently amended in such a way as to manifest an intention that it was not intended to exclude the operation of the State law, the operation of the State statute would thereupon revive. There is therefore no reason to doubt (assuming the correctness of the assumptions to which I have referred) that after 19 June 1983, when the Amendment Act came into force, the Anti-Discrimination Act again became operative in New South Wales.
In Viskauskas v. Niland , we held that s.19 of the State Act, making racial discrimination in relation to the provision of goods and services unlawful, and other provisions to the extent to which they relate to s.19 and to complaints for breaches of that section were inconsistent with the Commonwealth Act and to that extent were invalid by reason of s.109 of the Constitution. The basis of our decision was that the Commonwealth Act manifested an intention to occupy the relevant field to the exclusion of any other law. We indicated, without deciding, that we considered that the whole of Pt II of the State Act, dealing with racial discrimination, was inconsistent with the Commonwealth Act.
The subsequent amendment of the Commonwealth Act which inserted s.6A was designed to enable the Commonwealth Act and the State Act to operate side by side. It sought to achieve this result by specifically providing in s.6A that the Commonwealth Act:
" ... is not intended, and shall be deemed never to have been intended, to exclude or limit the operation of a law of a State or Territory that furthers the objects of the Convention and is capable of operating concurrently with this Act."
The source of this suggested invalidity is s.109 of the Constitution. Although the object of this section is to secure the paramountcy of Commonwealth laws over inconsistent State laws, the effect of the section, like that of other provisions of the Constitution, is to inhibit the Commonwealth Parliament from enacting contradictory legislation. The Parliament could not, for example, either prospectively or retrospectively provide that a State law which was inconsistent with a Commonwealth law should have, or have had, full force and effect, notwithstanding that inconsistency. This is because the invalidity of the inconsistent State law is brought about by the operation of s.109; the Commonwealth law does not operate of its own inherent force to invalidate the State enactment.
But there is no objection to the enactment of Commonwealth legislation whose effect is not to contradict s.109 of the Constitution but to remove the inconsistency which attracts the operation of that section. So, where inconsistency between Commonwealth and State laws arises, as it did in Viskauskas, because the Commonwealth law, according to its true construction, is intended to regulate the subject matter exhaustively or exclusively, the Commonwealth Parliament may legislate to remove that inconsistency by providing that the Commonwealth law is not intended to regulate the subject matter exhaustively or exclusively, thereby opening the way to the concurrent operation of a State law on the subject matter. It is, of course, well settled that:
"... a Commonwealth statute may provide that it is not intended to make exhaustive or exclusive provision with respect to the subject with which it deals thereby enabling State laws, not in direct conflict with a Commonwealth law, to have an operation ...".
 Section 109 of the Commonwealth Constitution provides that: ’When a law of a State is inconsistent with a law of the Commonwealth, the latter shall prevail, and the former shall, to the extent of the inconsistency, be invalid.’
 CENTRAL NORTHERN ADELAIDE HEALTH SERVICE v ATKINSON  SASC 371 (24 December 2008) per GRAY J
 (see Federated Saw Mill &c. Employes of Australasia v. James Moore & Son Proprietary Ltd.  HCA 43; (1909) 8 CLR 465, at p 536; Wenn v. Attorney-General (Vict.) (1948) 77 CLR 84, at p 120; Reg. v. Railways Appeals Board (N.S.W.); Ex parte Davis  HCA 22; (1957) 96 CLR 429, at p 439)
see the discussion by Mason J. in Reg. v. Credit Tribunal; Ex parte General Motors Acceptance Corporation  HCA 34; (1977) 137 CLR 545, at pp 562-564 and Palmdale-A.G.C.I. Ltd. v. Workers' Compensation Commission (N.S.W.)  HCA 69; (1977) 140 CLR 236, at pp 243-244)
 Wenn v. Attorney-General (Vict) (1948) 77 CLR 84, at pp 119-120; Reg. v The Members of the Railways Appeals Board and the Commissioner for Railways (N.S.W.); Ex parte Davis  HCA 22; (1957) 96 CLR 429, at p 439; Reg. v. Credit Tribunal; Ex parte General Motors Acceptance Corporation  HCA 34; (1977) 137 CLR 545, at p 563)