Green’s Dictionary of Slang

wacker adj.

also wacka
[whack v.1 (1)]

(Aus.) excellent, wonderful.

[Aus]A. Seymour One Day of the Year I i: The place is full of ’em. Isn’t it? Wacka!
[Aus]W. Dick Bunch of Ratbags 30: He hadn’t seen the mallee roots. Wacker, I thought.