I've been on the hunt the last two weeks for a 2nd hand DH rig and i used the below, i'm also selling my current bike on the below sites:
BikeExchange
Good place to pickup ex-demo's and new sales, even some decent 2nd hand rigs on here as its a paid to advertise site
Gumtree
Lots on here but as with all free to sell sites its buyer beware and make sure you can view the rig before you buy
Facebook Group Pages:
I see your in Melbourne, here are the ones i use:
VIC MTB Trading Post
Buy, Swap, Sell Downhill Bikes Victoria
Enduro Mtb Buy Swap And Sell Australia
MTB buy, swap, sell Australia
Local Distributors, don't discount these guys, unless your looking for a very pre-loved item they usually have something that was ex-demo or even a trade-in, both of my bikes i have got are from distributors and both were ex-demo or ex-team bikes, usually very well looked after and great price for the gear
Pinkbike, even though its a US site there are a few bikes on there for AU/NZ people, just check the descriptions to find out if the price quotes is USD or AUD