There are two kinds of jellyfish in Northern Austalia which can kill you in minutes, the box jellyfish (or sea wasp, stinger, etc), and the irukandji jellyfish.
The box jellys hang in quite close to shore, and are very rarely out on the reef, even in the middle of the wet season. However they are inshore on the coast from about october to march each year. A marginal sting will leave you in excruciating pain, but you can get through it. A really serious sting can kill you in under 10 minutes.
The good news is that all beaches have some vinegar handy (this neutralizes the stingers), and you can protect yourself against stings simply by wearing any sort of lycra suit or covering. The basic rule is that it's safe if you cant see through it. Lycra bodysuits are all you need for warmth anyway, so it's no big hassle.
The irukandji is a nasty little critter; they are tiny (one of my friends gor one down his snorkel on a field trip) and cause about 3 weeks of incredible pain. The same precautions apply as for box jellies. I wear a lycra suit, gloves (needed to stop coral cuts anyway), boots (for fins anyway), and sometimes a lycra hood (nicely helps with sunburnt ears!). That means the only part of my body that can be stung is that tiny line between snorkel and mask, and i have never been stung.
So, yes there are 2 jelly species that can kill you, BUT given some common sense precautions and a little dilligence, the chances of you getting stung are tiny.
At any rate, they aren't out on the reef.
), do you know which ones swim in your area?
... But how long would you say the the largest bicolors are (nose to end of tail)? And what length do you see most of? 
You not only have great subjects, but you are a great photographer.
