Prime Minister John Key says Kiwis are up for a debate about the flag.

The popular Red Peak Flag could be easily added as a referendum option, a law expert says, despite it being rejected by Prime Minister John Key.

Speaking on the Paul Henry show on Monday morning, Key guaranteed the flag, designed by Aaron Dustin, would not be a fifth option for New Zealand.

"To accept any other flag we would have to change the law and we're not going back to Parliament to change the law," he said.

SUPPLIED The 'Red Peak' flag will not be an option in the flag referendum despite growing support for the fifth flag alternative.

However Electoral law expert Graeme Edgeler said it would not be difficult for the government to add Red Peak to the ballot, if it wanted to.

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"They can't just add a fifth flag, but if the Government wanted to it could change its mind really quite easily," he said.

1 of 6 ALOFI KANTER Alofi Kanter's black and white fern design was announced as the first finalist. 2 of 6 KYLE LOCKWOOD Kyle Lockwood's red, white and blue design has made the final cut. 3 of 6 KYLE LOCKWOOD A second design from Kyle Lockwood has made the final cut. 4 of 6 Andrew Fyfe's koru flag was the third to be announced. 5 of 6 MONIQUE FORD/FAIRFAX NZ Deputy Prime Minister Bill English kicks off proceedings at the unveiling of the final four flag designs. 6 of 6 MONIQUE FORD/FAIRFAX NZ There's a fair bit of media interest at the unveiling of the final four flag designs at Te Papa.

"Cabinet could just issue a new order in council and change the alternative flags if it really wanted to."

But Edgeler didn't think that was likely unless an extraordinary groundswell of support for the flag was shown.

"Perhaps if there were a million people in a march, but I think it's highly unlikely anything could change the Government's mind in time for the printing of ballot."

ACT leader David Seymour echoed the advice, saying on Monday morning that there was nothing to stop Key and the Cabinet from making Red Peak one of the four.

"They needn't change the law, in fact they could do it today with the stroke of a pen by Order in Council," he said.

Red Peak should be one of the four options New Zealand voted on, Seymour said.

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In a Facebook post he said that even people who wanted a flag change had been "underwhelmed" by the final four choices, and Red Peak had had more "positive buzz over one weekend than pretty much the whole flag change process to date".

Substituting Red Peak for one of Kyle Lockwood's "nearly identical" designs was "the best chance of making this whole process worthwhile".

"Even if Red Peak doesn't become the national flag, it will give thousands of people something they can feel good about voting for.

"It's clever, it's original, it's simple yet distinctive, and it tells a story that is about New Zealand," Seymour added.

Support for the geometric Red Peak flag has grown rapidly since the final four flags were selected last Tuesday and support centred on the Red Peak design on Friday.

By Sunday a petition had emerged, begging Key to add the Red Peak flag to the referendum.

Within three days of launching, the petition had more than 16,800 signatures and by Monday morning, it had 20,900 signatures. Its aim was to reach 25,000.