Green’s Dictionary of Slang

(Aus.) excellent, splendid, first-rate.

1943		Albany Advertiser (WA) 21 Jan. 5/2: No doubt it would be very nice if the Council could ‘meet in committee’ - as a radio artist put it: ‘Whacko the diddle oh’ .
1949		R. Park Poor Man’s Orange 186: Of course, she wasn’t what any fellow could call a whacko-the-diddle-O piece, but she was a girl.
1977		S.L. Elliott Water under Bridge 313: Look, it’ll be whacko the diddle-o, I’m not worried [AND].