Mat
There are some good GTs available off brisbane, but you do need a few things in your favour. Structure, bait and current are important anywhere, but more so at the southern end of their range. The upside of fishing around here is that the larger fish seem to cope better with temperature extremes off the coast. Most of the fish I have seen locally have been upwards of 20kg and there are a fair number of 40+kg fish as well. The numbers of fish aren't that great, but is very rewarding to nail some fish on your home turf.
Some of the areas to look include all the shallow reefs from Cape Moreton to Hutchies, off Pt Lookout and the shallow reefs off the Tweed. Try places where reefs drop from 10-15m down to 20-40m and if you can combine it with some current and bait then hopefully success is not too far away. Another area I'd like to try on the sunshine coast is up at the far northern end of the Barwon Banks. The reefs are a bit deeper but when the currents run hard on the coast, it would be a very interesting place to fish.