Green’s Dictionary of Slang

(N.Z.) a fight, a brawl.

1909		Sun. Times (Perth) Supp. 19 Dec. 25/6: He was in the midst of training for the great soiree opf slatherurnwhack.
1916		‘Anzac’ On Anzac Trail 100: Before we got shut of it the battle [i.e. a street fight] had developed into a first-class slather-up .
1916		‘Anzac’ On the Anzac Trail 114: [of Gallipoli] Some of the chaps [...] laying odds on the chances of the takers [of bets] getting through the slather-up unharmed.
2003		McGill Reed Dict. of N.Z. Sl.