If NSW does it like they do here in the ACT what you have to do is (in neutral) rev the car to 4500rpm and if the dB are under the acceptable ammount then you pass and the exhaust is legal, if not it's not.
As far as 'kinds of exhaust' goes the only legal issues are: they must have a catalytic converter, they must fall below the dB level and they must not have an illegal baffle or restricter plate.
Given what exhausts do there arn't really different kinds, the differentiation is pretty much on size and generally speaking the larger the louder. There are less restrictive cats which might eke out an extra couple of hp, there are straight through mufflers which will reduce back pressure (important in a turbo application) however will make it quite loud. Other than that the difference lies in how pipes are bent - well if they are of if they are welded joins.
If you want the loudest exhaust possible then you're probably looking at 2" extractors, 2-3" from the cat and a 5" canon style muffler.
As a warning though if you do this to an excel it isn't going to impress anyone......
As an indication of the hassles exhausts can be a make of mine has a series 2 Rx7 with a turbocharged series7 engine he has something like a 3'' system with four mufflers (straight through though) and failed because it was too loud. The guys at the exhaust shop made up an illegal baffle that cut the exhaust to effectively 2'', coupled with the warning "if you get caught with this we didn't give it to you". So down to the pits again and he got lucky - revved to 4.5 and failed but a young guy was on and got him to try again at 3.5, this time he was under. As a warning though it chews prodiguous ammounts of fuel and he can't get insurance on it.....