Speaking of venomous Australian marine creatures...
See those tiny little bastards? That's Carukia barnesi, the Irukandji jellyfish, native to (as many venomous things are) Australia. If it's poisonous, deadly, and/or vicious, it lives in Australia. Or Africa. Or the southeastern United States.
Big surprises come in small packages, right? At 12mm x 25mm in size, this sucker packs one hell of a wallop.
How big? Well, its (contractable) tentacles range from 5 cm to ONE METER in length. A scale image (VERY LARGE PICTURE WARNING) of the jelly and its horrifying tentacles. Did I mention that its nematocysts/stingers aren't just on its tentacles? Nope, on the bell, too.
What'll happen if you get stung? Haha, well...
You can die. Two reported cases have been attributed to Irukandji Syndrome (the resulting symptoms of the stings), but it's unclear how many may have been falsely attributed to other factors.
Mostly, you end up writhing in excruciating pain for anywhere from 4 to 30 hours, and residual symptoms of the syndrome (aches and pains, mostly) can stick around for up to a week afterwards.
I've actually seen the Discovery Channel presentation where two researches accidentally got stung. There was a piece of tentacle stuck to the lady's wetsuit, and it fell on her skin. The guy touched it when he flicked it off of her (or something like that).
Even under the "maximum dose of morphine" Teresa (Carrette) remarked that she "wished she could rip her skin off", and is later seen writhing uncontrollably from the pain, while lying on her hospital bed. In a particularly disturbing shot, we see Teresa's feet contorting and digging into the bed. When the camera pulls out to a wide shot, she is rubbing her face, her body is contorting in agony, and her legs are rapidly sliding and kicking around on the bed. Jamie (Seymour), at his worst, is also seen writhing in pain, curled up in a ball and barely able to speak. Jamie said he wished that he was stung by Chironex fleckeri instead since "the pain goes away in 20 minutes or you die".
Wiki
Great Barrier Reef Research Centre