Green’s Dictionary of Slang

(Aus.) a coffin.

1885		Bulletin (Sydney) 28 Mar. 20/1: Then followed the funeral reform alluded to. The body was put in a ‘piner’ laid on a hand-truck, and given over to four prisoners and a warder. These men had a rare old time with the ‘stiff ’un.’.
1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 114: PINER: a plain deal coffin as distinct from a more elaborate polished cedar or oak one.