Green’s Dictionary of Slang

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

1961		B. Crump Hang On a Minute, Mate (1963) 163: When he’s got a few in he goes berko.
1988		McGill Dict. of Kiwi Sl. 14/1: berko berserk, though not really; to go berko is to be temporarily angry and out of control.
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] [I]f the crowd’s view kept being obstructed and there were one or two bad decisions, they might go a bit berko and start throwing chairs along with the abuse.
2003		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.