I noticed TJ referenced Walters as a source for N scale locomotives for you.
Here is a online source I used in the past for model railroad equipment.
They are a discount online store, and their listed prices are cheaper than Walters if they are bought in the U.S. I don;t know if that would hold true for Australia though, but I think it would be worth your time in checking it out. The second one is also a discount online store too.