That's the sea wasp, also known as the box jellyfish. There are multiple types, but, believe it or not, the tiniest ones are the most deadly and most painful. Their main area is, in fact, the waters around Australia. That is why people often wear wet suits that look like pantyhose over their entire bodies before going swimming in the ocean. Because the animals will sting ANYTHING living. But when in contact with even thin cloth, they don't sting because their stingers don't register it as a living being.