I caught a few of those as a kid in rock pools by the beach. They'll hurtcha, but the box jellyfish is much deadlier, it has the only known toxin to attack the circulatory and respiratory systems simultaneously, according to what I've read. In the Northern parts of Australia you can't swim in the ocean at all for the hottest half of the year because you're pretty much guaranteed to get stung during those times, it's much safer swimming in the rivers with the crocodiles.
Australia has quite a few poisonous creatures ... 7 of the worlds 10 most poisonous snakes live in Australia for example. Brings a whole new meaning to Aussie Aussie Aussie, Oi Oi Oi ! :eek: