JELLYFISH STINGS

JELLYFISH STINGS (Chironex Fleckeri and Chiropsalmus Quadrigatus)
The sting of the large box jellyfish, found along the northern tropical coastline of Australia, can be deadly. The venom injected by these animals is capable of killing humans within minutes.
When a sting occurs, pieces of tentacle may be left in the victims skin which can continue to release venom. However, by dousing the sting area with regular vinegar (5% acetic acid content), the further release of poison will be rapidly and completely stopped.
That's why Australian lifeguards always keep vinegar on hand and it's their initial first aid treatment of choice.

Reference:
Australian Resuscitation Council Guideline 8.9.6