Green’s Dictionary of Slang

(N.Z.) naked, esp. in the context of swimming.

1940		Marlborough boys (Ed.) Go swimming in the nick [DNZE].
1986		T. Winton That Eye, The Sky 43: Here I am in the nick, raw as a prawn.
1989		Listener (N.Z.) 20 Nov. 36: She’d seemed so unaware of her nudity. The mother of his Pakeha friend, in the nick [DNZE].
2003		McGill Reed Dict. of N.Z. Sl.