hi buliej,
this is an interesting issues - i chart on a US site and some of the discussions have touched on this topic - there is generally an incredible response when an Aust person posts that they are only putting one or two back!

I'm not sure of the science and stats from the US of putting back 3+ embryos, so wont even go there. However, some of the 'folklaw' points that I have picked up from the chat rooms (so take them with a large grain of salt as I can not vouch for how accurate this is), are...
1. US doctors are encouraged to put back more embryos to reduce the number of cycles that it takes to fall pg, even thought the risk of multipules is significant. This is (or at least started out being) driven by the private insurance companies who wanted to pay for less cycles as IVF is not covered by public insurance.
2. So, the downside is that embryo reductions are performed in the US (despite their oposition to abortion), as an accepted part of fertility treatment to reduce the number of living embryos at an early stage in the pg to give the remaining embies more of a chance to make it to full term.

As I said, I can not vouch for how accurate this is, nor if it applies to the 40+ age group, but it may provide some cultural context to why they may have a different approach.

In terms of what is or is not possible in Vic, I think that the source would be The Infertility Treatment Authority (the statutory body established by the Victorian Parliament to administer the regulation of infertility treatment within the State of Victoria, Australia. It has been established under the provisions of the Infertility Treatment Act 1995). It has a website. If you wanted to look at the specific legislation, working through the website of the Vic Parliment should get you to the specific Act.

Good luck with your research!
FG