This will be useful to WA readers, for other states don't know.
Replaced a faulty smoke alarm (rental property, had no option), and wondered what to do with the old one. The brochure included sternly worded disposal requirements which included sending to the health department, or sending to a specific address in Victoria. So I contacted the health department in Perth, who kindly reverted promptly with the following: As long as the detector uses < 1.0 uCurie of Americium241, it can be disposed of with normal domestic rubbish. I haven't checked, but just about all current models are likely to be in this category. If you remove the cover of the device, there is a label on top of the ionisation chamber stating the material and quantity.