Australia Beaches

Best time to go:
South East Coast [Sydney] - Central East [Brisbane/Gold Coast]: Oct-April for swimming
North East Coast [Cairns & Great Barrier Reef islands]: all year is hot [above 26C] but Jan-March gets some rain and December is crowded and super heated, so April - Nov is best for swimming. Scuba divers like Sept-Dec.

Australia is blessed with too many amazing beaches to list, offering some of the world's best surfing, snorkelling, scuba diving, and of course just posing around on soft sand chugging a cool one. In the right season the sun is hot, the sea is cool and the beer is cold. Heaven. However....
The country is also cursed with a staggering collection of deadly creatures, on land and at sea. Not very common, these critters are well-known to locals and treated fairly casually, but visitors used to less hazardous waters should be aware:

Watersports hazards:
Precautions:
Don't touch anything you don't recognise as safe!
Consider sand shoes for paddling around beaches and a light Lycra stinger suit or wetsuit for snorkelling or scuba diving if you have a weak heart and are in a stinger zone, such as north of Brisbane.
Swim inside stinger nets when in the zone.

Seriously nasty:
Box jellyfish: in the Oct-May season, wear a Lycra 'stinger suit' or wetsuit and keep your eyes peeled to avoid this deadly Mr Blobby. Usually found in deep water well off Australia Beaches so a problem mostly for snorkellers and divers.
They appear to spawn around the Barrier Reef and like warm, no-surf waters so the Reef and north of it are danger zones.
Irukandji jellyfish: the new tiny terror [peanut sized] that prefers deep water but can be swept through [anti] stinger nets by currents.

Blue-ringed octopus: small, cute and occasionally fatal, even when it's washed up on the beach or frolicking in a rock pool. You wouldn't be so dumb as to play with the little chap, but the kids would.

Salt water crocodiles: far more dangerous than sharks 'salties' hang out where rivers meet the sea, so however hot and sticky you are be extremely cautious about swimming in rivers or in/around estuary beaches, especially if no one else is there or there are warning signs. Freshwater crocs in Australia are not a problem, and since they are eaten by salties too, if they are around then salties probably aren't.

Not nice:
Cone shells: often host a snail with a noxious needle that has killed.

Stonefish: almost invisible tucked under sand, their poisonous spines are very potent and may mean a hospital trip. Hot water will disperse the toxins. Sand shoes or flippers will usually prevent penetration.

Sharks: overrated in the danger stakes due to bad PR, attacks are usually a case of mistaken identity, when a shark - confused by waves - thinks a surfer is a seal. You have more chance of being killed by a falling coconut.

Don't be intimidated in spite of all the above, Aussies aren't! If you keep your eyes open and take reasonable precautions you'll have a great time.
The Bugbog crew did get wrapped by a 'Bluebottle' [Portuguese Man-of-war] while diving off the Barrier Reef near Cairns. No big deal - a pint of vinegar and an aspirin sorted it out.

Protection: popular beaches are well provided with lifeguards, safety flags, warning signs and stinger nets where necessary. They are there to protect you so look for their guidance and stay within recommended limits.

Clubs: Some of the best spots on Australia Beaches are occupied by clubs. These offer great value food and drinks as well as superb views and are usable by foreign visitors, so take proof of foreign residence [more than a passport, papers with your name and address are needed] and you will be hosted by the best place in the area.

The sun: There is no point lying around in the sun at midday in Oz. This will not only earn you a place in the melanoma sweepstakes, give you wrinkles and sagging skin at an early age and add an unpleasant red highlight to your tan, but it will probably burn the tan off altogether.
You will brown up more smoothly and lastingly by sunbathing before 11am and after 3pm. And you may live longer too.