New Zealand First's leader won't say how long it could take for them to strike a coalition deal if they hold the balance of power following the election.

National has sparked fears there could be a long negotiation period if they can't form a Government with ACT alone.

Winston Peters says they have a responsibility to deliver a strong, stable Government.

"The reality is, if it takes time to get it right - say, it takes 12 days - well, that'll be fine."

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