Green’s Dictionary of Slang

kotanga n.

[pun on pron. SE coathanger, sometimes used as an ad hoc car aerial]

(N.Z.) a car aerial.

[NZ]McGill Reed Dict. of N.Z. Sl. 124: kotanga Car aerial, a pun on ‘coathanger’, which is sometimes twisted out into a diamond shape and employed as an ad hoc car aerial.