You'll need to go a bit further than the shelf.
Out of sydney - they are being caught just past browns. So you're looking at about 30km out - keep in mind that you don't just drive to that 30km, pull up and catch a tuna though. You get to that 30km and zig-zag around for a while until you find something.
A few fish are being caught around Southern Canyons too, on an ideal day I'd probably go down to the southern canyons and troll up to Browns.
BUT the most important thing to remember is that there has to be fish there to catch, so you can make the trip out and end up with zero's - which is what happened to 50 odd boats at batemans last sunday.
Start reading up on Altimetry and Sea Surface Temperatures, this will give you the best idea of what the fish are doing (next to first hand reports).
How far offshore have you been previously?