Melbourne has the big 4 plus a wide range of medium tier firms. And has many many small firms too. If you wanted to work in commerce, that's a bit dicey ATM given the GFC.
For an idea of salary, go to
SEEK - Australia's no. 1 jobs, employment, career and recruitment site
and work your way from there.
Australia has 2 main professional bodies:
1) CPA Australia
2) Institute of Chartered Accountants
Both are in competition with eachother for members, and both have tried to merge, but the members voted against it.
But in either case, you're required to do about 60 PD hours in a triennium.
The Australian accounting profession is considered one of the most stringent in the world, probably second only to the USA. They are hot on things like ethics, conflict of interest and governance. The Australian Accounting Standards Board is harmonising the Australian accounting standards with international standards so if you've ever had to apply international standards, the Australian standards shouldn't be too foreign to you.
As for Melbourne itself, nothing much to add that hasn't been already been stated elsewhere on this site. Great city, growing very quickly, in the midst of a population boom, very multicultural, plenty to see and do, pretty good lifestyle but has its issue with petty crime as most cities do.
Can't comment on Toronto as I've never been.
Good luck with your decision, either way.