I still remember the first real shark I saw spearfishing, I was in a 15m deep channel and saw this streamlined shape, absolute unmoving streamlined perfection gliding past me, the tail and fins weren't moving a millimetre, but somehow it was cruising along very comfortably. My first instinct was to panic, then the rational part of my brain took over about half a second later:
"If you swim for shore, then it can beat you with no problems whatsover. If you sit still then it
might swim away..."
So I extended my speargun, and charged at the shark as fast as I could. Fight fire with fire, I say! My last image of it is the eye (which looks very much like a cats eye) looking disdainfully at me as it disappeared in the murk, fins still not moving while I was thrashing wildly trying to keep up with it.
Since then I've swum with bigger sharks, had sharks buzz me when trying to knife a speared salmon (charge at the shark, keep the fish near and it won't take your hard earned dinner!) and watched sharks take the odd fish or two.
Sharks mostly bite things out of curiosity, the more I watch them the less I'm afraid of them. As other people said, they're a lot like puppy dogs. Just watch for the danger signals: Back arched, pectoral fins down means the same as ears back and hackles raised. In that situation it's a good idea to get out of the water.
Now as for grey nurse shark protection zones... as far as I can tell they're more a political ploy than anything else, the number of GNS sightings by spearfisherman far outweighs the "official population", it's gotten to the stage where if you see a GNS then you daren't tell the authorities because then they'll slap a 1km exclusion zone around it, and you won't be able to spearfish there anymore. Scuba divers do far more harm to GNS's than spearo's (camera flashes, grabbing onto coral, going down into the GNS cave to get a close look at the little 'uns etc) yet most exclusion zones allow bubblies yet no spearo's?!?
Sorry for the rant