Personally I'm more wary of the jellyfish than sharks - the beach was blue with them the other day as the tide went out and I remember the same scenario from last year. They're little blue ones - they're pretty common here and I think they're called bluebottles - but don't let their size fool you, apparently the sting hurts.. alot.. heard a few stories from surfers when I worked at the shop, seems vinegar is the thing to use if you're unlucky enough to be stung so head to your local chippie!!
I've only seen them on Sumner and South Shore, not sure how widespread they are.