Rips, not sharks
Rip currents are the leading cause of tourist deaths in Australian waters and kill more people than sharks, crocodiles and snakes combined. A rip looks like the calmest patch of water — darker, flatter, less foam — which is exactly why swimmers choose it.
Swim between the red and yellow flags on a patrolled beach, and only there. If caught, do not swim against it: float, raise an arm, and let it carry you out until it releases, then swim parallel to the shore.
In the tropical north, box jellyfish and irukandji make the sea unsafe from roughly November to May. Stinger nets and vinegar stations exist for a reason, and saltwater crocodiles inhabit rivers and estuaries well inland — obey the signs absolutely.
Sun and distance
Australia has among the highest skin cancer rates on earth and a UV index that reaches extreme levels in a way northern visitors have no intuition for. Burns happen in under fifteen minutes in summer.
Driving: outback routes run hundreds of kilometres between fuel with no mobile coverage, kangaroos are most active at dawn and dusk and cause serious crashes, and road-train trucks need enormous overtaking room. Carry water — vehicle breakdowns in the interior have killed people.