Labour's finance spokesperson has apologised to the Finance Minister after audio of her calling Nicola Willis a "duck-faced horse" was leaked to media.

Barbara Edmonds told RNZ she got it wrong, and feels "absolutely terrible about it."

The comment was made during a question and answer session at a Labour Party candidate list conference at the weekend, and was designed to test MPs about difficult questions that may be put to them, or questions that didn't make sense.

During the exercise, a random question was used that might be put to an MP, and the task for candidates was to respond in a way that may allow them to present Labour's political messaging instead.

The question posed in the exercise was "would you rather fight 100 duck-sized horses or one horse-sized duck?"

When it was her turn to respond, Edmonds said "every week I have to stand up in the house and ask a duck-faced horse - did I get that right [laughs] - questions every single week."