Hi there,
It depends where you plan to swim. Personally, I didn’t go into any creek in NT~our guide (a local aboriginal girl) out on a trip told me about her sister getting dragged into a culvert by the leg by a big old saltie just lying there waiting for anything with a pulse to wander by. Had it not been for her husband, she would have been in real trouble. I’m not so desperate to swim!
With that in mind, do beware of the saltwater crocodiles near inland water in the northern part of the country, and don’t go mucking about close to the edge. There will be signs about too~just not everywhere. Australia is a big place.
There are box jellyfish around parts of the coast some of the time too. And there are sharks, but if you swim on patrolled beaches within the flagged areas, you will be fine.
There all kinds of creatures that can bite and sting and eat you in Oz but just remember that you’re more likely to be killed by a horse than any of them.
If you want to read a bit more:
Natural Australia
Swim and have fun. The Aussies love the water.