Green’s Dictionary of Slang

(N.Z.) things have started! now we’re getting down to business!

1941		Baker N.Z. Sl.
1943		J.A.W. Bennett ‘English as it is Spoken in N.Z.’ in AS XVIII:2 Apr. 90: The exclamation ‘They’re off, Mr. Cutts!’, announcing that any contest or affair has begun, contains the name of a one-time popular starter at the Auckland races.
1988		McGill Dict. of Kiwi Sl. 112/2: they’re off, Mr Cutts the race has begun; Partridge identifies as a variant of ‘“they’re off,” said the monkey’, usually indicating start of a horse race, hence as something that has come loose; often with addition: ‘when he looked into the lawnmower and carelessly lost his testicles.’.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].