Green’s Dictionary of Slang

(N.Z. prison) any form oof a prison strike.

1978		G. Newbold ‘Social Organization of Prisons’ in dissertation U. Auckland 341: [O]pinion was divided between a more militant faction who advocated a ‘prop’ and another group who wanted to ‘cop it sweet’ [...] Everyone knew that a prop would very likely end in a Riot Squad baton charge.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 146/1: prop n. a strike, which may take several forms; from a peaceful protest, e.g. a group refusal to obey officers’ orders, or a sit-down strike, to a protest incorporating extreme action of some kind, e.g. a riot.