Green’s Dictionary of Slang

/N.Z.(Scots) a single night’s imprisonment; a very short sentence.

2009		L. McIlvanney All the Colours 320: There are minor arrests [...] the odd overnighter in the cells.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 132/2: overnighter n an inmate serving a very brief sentence.