Blackmax, I'm guessing your targeting southern bluefin? I've never fished for these but I have caught alot of yellowfin and from what I have read SBF are caught the same way.
I would start with a spread of pushers. I would imagine 10inch lures to be the best size. There are lots of good lures but I would recommend Pakula meduim sproket, Pakula cockroach, Topgun meduim nightmare, Blackbart mini 1656 angle face etc. These lures are easy to run and will work in most sea conditions.
Run darker lures close to the boat and brighter or lighter colours farther back. I would recommend nightmare in purple or black short corner, 1656 in blue colour long corner. Cockroach in a pink clour short rigger and sprocket in lumo off the long rigger.
If your unsure about what I mean when talking about the postions short corner etc then log onto the Pakula website as there is some good info there about setting up a spread of lures for trolling.
As for troll speed you generally want to be doing around 7.5knots but this will depend on the weather on each given day. You want your lures to breath about every 5 seconds. When I say breath I mean come to the surface and splash before diving again and making a smoke trail of bubbles. Speed up or slow down until this is achieved.
I don't know what rods you are planning to fish with or what size fish you will be targeting but I would image a spread of 24kg outfits to be a good meduim capable of catching any bigger fish that might climb on. You will have to run 50 size reels and quality rods. If your serious about targeting big tuna then invest in bent butt rods as tuna tend to fight deep which can be hard on your back.
I don't know the coast of Tassie at all so I would recommend you look at some charts and locate some dropoffs, canyons, seamounts etc. I don't know how far it is to the start of the shelf but this would be a good area to start. Take a look at some sea surface temp charts on the net to find where the warm water meets the cold. this is where the schools of tuna will be. The latest Bluewater mag has a good write up on how to read these charts correctly.
I hope this helps you. If you have any questions just ask.