The issue with large fish (like flake, tuna, marlin, etc) is that they can contain higher levels of mercury than small fish, simply because they eat little fish and will have a higher concentration in their system. That said, most Australians don't eat the recommended 3-4 serves of fish a week, so if you are having the odd bit of flake or can of tuna it is probably not going to cause you any harm. It also helps if you try and source Australian/NZ waters fish.

If anything, the prawns from a local fish 'n' chippie are likely to be nasty imported stuff from some dodgy Asian fishing ground, so they would probably be far worse than any large fish on offer.