I just did another check and apparently the adults come up river to give birth, so at any time there could be bigs ones in the water. But, not sure how far up they come. The other shark question I posted an article link.
Bull sharks do head up the Swan River, but they should mostly be small ones! They head back out to the ocean as they grow up a bit. I think they could pose a risk to small kids and pets, so don't let them in the water in the early or late feeding hours. I have seen one take a pelican sitting on the water before - a lot of splashing and bird disappeared pretty quickly! Quite amazing!