Parliament has gone into urgency to pass legislation adding a fifth design to the flag referendum.
The change has come after a swift political move by the Greens - ending a standoff between National and Labour.
The Greens had tried to introduce the bill by leave of Parliament, which would allow the Red Peak design to be added to the existing line-up.
However, that was blocked by New Zealand First, which said it opposed any change to the current flag.
Prime Minister John Key said his office had an approach from the Greens and, accordingly, National agreed not to block the members bill this afternoon.
It was introduced under the name of the minister responsible for the flag referenda, Bill English, and debated this afternoon under urgency.
Red Peak has gained popularity in recent weeks, and a petition with about 50,000 signatures calling for it to be added was delivered to Parliament last week.
But during Parliament's general debate, New Zealand First deputy leader Ron Mark likened it to that used by the Nazis on their sentry boxes during World War II.
"Give me a break, if there is one further insult that Mr Brownlee, the Minister of Defence, can serve up to the veterans of this nation, is to offer them up a Nazi logo, a Nazi sentry box as the national flag, you have got to be kidding me.
"Mr Speaker, we were asked coming into the Chamber, are we prepared, New Zealand First, to stand alone, to be the only party in this House to oppose, one, the referendum, two, the changing of the flag and, three, the introduction of this Green Party, National Party Nazi flag.
"We are quite happy to be the only party to stand up in defence of the New Zealand flag."
The Labour Party has already unsuccessfully tried to introduce a bill adding Red Peak to the referendum but that bill would also have changed the questions asked in the first referendum - a change that the Government said it could not support.
Mr Key said the Greens would not support Labour if it tried to introduce that amendment today.
"Well, the bigger issue really is that if a bill gets introduced either by us or the Greens, the question is well what will happen if Labour put up a yes/no supplementary order paper, and the deal the Greens have done is they would vote that down, they would vote against Labour," he said.
Mr Key is defending his support to change the flag referendum process.
"The whole way through I've said my preference is to stick with the process that the officials gave us, stick with the four.
"I said I'd be prepared to go outside that but I just didn't want people to play games.
"The Greens have been very, very straight, here's a straight bill, we're not going to support any SOPs or amendments to try and go back to the sorts of things Labour's been talking about."
All about choice
Green Party MP Gareth Hughes said his bill was about giving New Zealanders a choice, following the groundswell of support for the Red Peak flag to be included.
"The ball is in the other parties' court. If it is blocked - and a single Member of Parliament can block it - I'm calling on the government to adopt it as a government Bill, because the important thing isn't who passes the bill, it's that we have a Red Peak choice in the referendum in six weeks time.
"This bill provides a constructive solution that gives people the choice of a flag that's managed to engage more people in the flag debate than any other part of the process.
Labour leader Andrew Little said his party tried to work with the Government last week.
"In the end this is a process that is now become so flawed, handled in such a shambolic fashion, all of the responsibility of which lies at John Key's feet.
'Whether or not he thinks he can rescue it or do something about it, that's entirely a matter for him - he can do something if he wants, so far he's chosen not to."