I think a travel agent may be beneficial.
You can buy all one way tickets, but you have to look really hard to get overseas one way tickets cheap and book ahead. A good site is Sidestep
or if you're a student, Student Universe
. They do one way tickets at regular OW prices. There are a few others, but I found those two to be the best.
You can also get a RT ticket from Toronto. Some sights (can't think of any offhand!) allow you to land and depart in two different cities (Tor-Bang, Syd-Tor). Then, you'd just have to get a OW ticket from Bangkok to Sydney.
You can start by pricing those out. If it seems affordable, I'd say book. If not, then hit up the travel agents. I'm sure they've dealt with many, many situations like yours.