Green’s Dictionary of Slang

(Aus.) a mythical breed of ultra-aggressive koalas that lurk in trees and drop on unsuspecting tourists.

1982		N. Keesing Lily on Dustbin 85: The ‘drop bears’ are creatures of a tall story - they were invented during World War II for the benefit of gullible American servicemen. It is alleged that ‘drop bears’ are a dangerous kind of koala and that they drop out of trees on the heads and shoulders of bush walkers and hug them to death.
1991		Woroni (Canberra) 21 Oct. 18/1: Just like in Australia where kangaroos hop down the urban center streets, where there’s no indoor plumbing, and where drop bears and bush chooks are the terrorists of the bush.
2004		J. Lambert Maquarie Dict. Aus. Sl. 70/2: drop bear a vicious breed of koala that supposedly leaps upon unsuspecting tourists and attacks with unmitigated fury. A tale told to frighten unwary foreigners since at least the 1960s.