that is exactly what you have to do if you don't happen to have a handy jar of vinegar on you.
Speaking as a part-time Aussie, seeing a bloke writhing in pain on the beach while his mates stand around pissing on him is a more common sight than you might think on some Australian beaches.
As a former pseudo-Aussie (or faux-Aussie, aka Fozzie by no one ever), I can humbly attest that this is an old wives tale not backed by anything save the placebo effect. Do not put vinegar on it either.
I think people just like excuses to be peed on in public.