Warm tropical waters of Northern Australia contain a multitude of marine life hell-bent on self survival. They do not run about attacking tourists, but they also live in a survival of the fittest environment, so if you impede on what they regard is home territory they bite or sting.
Summer months -December to April in the Top End, Broome, Darwin, and Cairns regions are the most commonly affected. Some undersea plants have a poisonous nettle touch and it hurts.