Killer Jelly Fish As if there were not enough things that can cause death in Australia, jellyfish can take you to another neighborhood too. Even the beach is completely safe. Box jellyfish are one of the most common beasts of jellyfish in Australia found. They are not friendly. Box jellyfish produce a powerful poison that can be very painful and deadly to humans. The Irukandji jellyfish is even more dangerous and is the size of a fingernail. These small send 50 to 100 people to the hospital each year.