Aaron, if the little guys are only about 30cm in lengh, they're not going to be in a position to attack a decent sized lure. A sinking stickbait will get down to them so it may be worth trying. Another thing is perhaps a big soft plastic. GTs actually really like these. If chuggers are not working, perhaps a big pencil popper might get them going.
As you've probably gathered, I'm not too proud to try anything to catch a fish if I can't get it the conventional way.
Unfortunately, sometimes, the damn fish are just not feeding. Anyone who has fished for a long time will have experienced those terrible times when you can see amazing fish but you just can't get them to feed! The one good thing to take from this is that you know the big fish are there and that they will feed sometime! Persevere and I'm sure you will succeed. GTs are simply too voracious a predator to stay quiet for long. In water that depth, I'd encourage you to persevere with chuggers. I am certain you will get one sooner or later.
You can imagine how envious you're making those of us who live in places like Sydney having access to areas like that!!