Green’s Dictionary of Slang

berko adj.

also berkers
[abbr. SE beserk + -o sfx (4)]

(Aus./N.Z.) berserk; in weakened form, temporarily out of control.

[NZ]B. Crump Hang On a Minute, Mate (1963) 163: When he’s got a few in he goes berko.
[NZ]McGill Dict. of Kiwi Sl. 14/1: berko berserk, though not really; to go berko is to be temporarily angry and out of control.
Aus. / NZ Jrnl Sociology XXVII n.p.: I just went absolutely berko on her.
[NZ]McGill Reed Dict. of N.Z. Sl. 20: berkers/berko Somebody considered excessively silly or angry, crazy or out of control [...] usually in the phrase ‘to go berko’. ANZ mid C20.