If you are lucky enough to find yellowfin tuna feeding on top of the water, you might want to try a skirted lure on top of the water, or a feather running either on or just beneath the surface.  If the water is very calm, you are going to get beneath the surface, and a deep diver or bibles minnow might be your best bet.  Just begin by trolling several offerings and see what is working.  Once you have determined what it is they are biting, you can adjust your fishing method and lures accordingly.  Begin by trolling with a couple of deep diving minnows on a flat line, and use small skirted lures on your outriggers, and keep a feather or bullethead lure at the center of your spread.
 
One more bit of advice.  There is nothing like a tuna for sport action, and nothing like it for fine eating, so whether you are looking for table fish or sport, just hang on for the run of your life.  These 'fin, as our friends in "Oz" call them are true fighters, and catching one is a thrill you are not likely to ever forget!

To get the full "Australian Saltwater Fishing - What To Catch And How To Catch Them" article you'll need to download it here.