[W]hen a law may possibly be regarded as having either of two subjects as its substance, one of which is within Commonwealth power and the other is not, a decision must be made as to that which is in truth the subject matter of the law. Although usually not an appropriate course in determining whether a law is a law on an enumerated topic, in such a case, the decision of what is the subject matter of the law may be approached somewhat in the manner the validity of a law claimed to be within one of the two mutually exclusive lists in the Canadian Constitution is determined. The law must be upon one or other of the subjects. It cannot be on both.
To the extent that the Chief Justice might be taken to have been suggesting that a law upon a subject within Commonwealth power may cease to be valid because it affects a subject outside power, these remarks have been disapproved: see the Tasmanian Dam Case [11] ; Queensland Electricity Commission [12] . On the other hand, if the relevant law has as its substance two subjects, one of which is exclusively within State legislative power, the observations of Barwick C.J. have greater force, as was recognized by Stephen J. in Actors and Announcers Equity v. Fontana Films [13] ; see also the Tasmanian Dam Case [14] . Indeed, that is the context in which his Honour's remarks were made.
1. (1971) 122 C.L.R. 353, at pp. 372-373.
2. (1983) 158 C.L.R., at pp. 150-152, 215.
3. (1985) 159 C.L.R., at pp. 260-261.
4. (1982) 150 C.L.R., at p. 194.
5. (1983) 158 C.L.R., at p. 152.