Keep trying mate and im sure you'll get one of the bigger fish soon, unfortunately so many aussie lakes contain hords of small ones, you just have to wade through them and sooner or later a bigger one will turn up.
Definately try boilies as mentioned as they do tend to sort out the bigger fish, especialy large baits (18mm+).
As for the morass youve mentioned i would just give it a go, if there are no signs saying dont fish then your on.
If your worried try and tuck yourself out of the way somewhere if possible, but i doubt you'll see anyone who will try to stop you. Whats the worst that can happen, they will only ask you to leave!!