Mate in QLD, I don't think its an issue, but ignorance of the law from the police prospective is a different story (left to local interpetation - that can be scarey). If I had a 30rd mag, I'd use it on a property, not on a range, so as to not draw the crabs to ones self.
I often see over capacity mags at the guns hows up here, but again this is QLD, Australia's Alberta.
NSW and VIC are two different kettles of fish, and I can't speak for the NT or the remaining states either.
Cheers,
Wes