2. Jellyfish

A box jellyfish.

IMAGE: MELANIE STETSON FREEMAN

Highly dangerous, the box jellyfish is usually found in northern Australia during the warmer months — exactly when you want to swim.

They're called box jellyfish because their bell — the top of the jellyfish — has four corners, as well as clusters of trailing, stinging tentacles that can stretch more than two metres (6.4 feet), Robinson said.

Human encounters occur most often when the box jellyfish comes close to shore to breed in estuaries. "That type of water is muddy, so you can't always see them and they can't see you," he added. If you're in the murky water and brush against one, you can easily be stung.

Danger rating: 10/10

Scientific name: Chironex fleckeri

How to know if it's got you: A box jellyfish sting can be unbelievably painful, Robinson said. The venom is designed to paralyse fish, so it immobilises your nerves and affects breathing and movement. A large dose can cause cardiac arrest and death within minutes.