The deal to save the Red Peak flag design will lose votes for all involved as Kiwis living outside the beltway of the political world do not care for it, says New Zealand First leader Winston Peters.

His warning came as the law was changed under urgency to add the design as a fifth alternative in the flag referendum, after an unlikely deal between the Green Party and National.

NZ First is strongly opposed to any flag change and its deputy leader, Ron Mark, this week compared Red Peak to markings on a Nazi sentry box.

Mr Peters arrived back yesterday after a trip to England for the Parliamentary Rugby World Cup, and expressed his contempt for developments.

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"If you think the Government is being smart here or the Greens are being smart, you have no idea what is going on outside the beltway," Mr Peters told reporters.

"If you think that an online, social media 50,000 [signature petition] - less than 1 per cent of our population - is in any way 'public mood' then, sadly, you have it wrong."

Prime Minister John Key said yesterday he would vote for Red Peak over the current flag, despite previously saying he was not taken with the design.

"I think if it got to that point I might vote for Red Peak, but I'm keen on changing the flag because there's a really strong argument about the future, my preference still very much so remains one of the Kyle Lockwood silver fern ones."