i was there in 2004.. icant remember how we did it but me and my wife signed on with two agencies just north of manly and they got us working in warehouses, one good place although very boring was a a vitamin tablet company that was clean easy work but just very very boring.. not bad paid either... look in the local papers and yellow pages (its not called yellow pages though!!)
in sydney itself - forget what street now but one of the main streets in the city - theres a travel place that helps all backpackers, go there for help (sorry thats not much help)
just remembered vip -
http://www.vipbackpackers.com/
backpackerworld is another but for me go to agency that employs aussies because they know english backpackers will work and not go surfing when its sunny!!