Green’s Dictionary of Slang

(N.Z.) a threepenny piece.

1943		J.A.W. Bennett ‘English as it is Spoken in N.Z.’ in AS XVIII:2 Apr. 91: In New Zealand slang a penny is a brown, a threepenny piece a thray (cf. English tray).
1972		(ref. to 1930s) R.P. Hargreaves From Beads to Banknotes 151: Threepence – thray.