Karl Chandler.
"G'day dickhead!" is a catchphrase of The Little Dum Dum Club, first uttered by Karl Chandler on the very first episode after being introduced by co-host Tommy Dassalo.
Originally said as a mock-insult on the fly, Chandler repeated the phrase during the introduction of the second episode, and has gone on to say a variation of the phrase in nearly every episode of the show.