John Key would back Red Peak if it made it to a run-off with the current New Zealand flag.
At a media stand up in Ashburton this morning, Mr Key said he would likely vote for the Aaron Dustin design should it make it through the first referendum.
"If it got to that point I think I might vote for Red Peak, but I'm keen on changing the flag because I think there is a really strong argument about the future. My preference still very much remains one of the Kyle Lockwood silver fern ones," he says.
The statement comes as Parliament voted under urgency 109 to 12 this morning to include Red Peak into the New Zealand flag referendum.
The Bill has the numbers to pass, and would mean the four shortlisted options will become five.
Parliament resumed its sitting under urgency this morning after politicians failed to complete the process last night.
The political deadlock between National and Labour over adding the flag and changing the referendum process was broken after the Green Party brokered a deal to get it included.
Green MP Gareth Hughes tried to introduce the Bill into the House yesterday but failed because it didn't get unanimous support – New Zealand First voted it down.
Mr Key agreed to adopt the Bill which then went through the Parliamentary process under urgency.
In the debate this morning, Mr Hughes says he was glad cross-party consensus could be reached.
"I'm glad parties in this Parliament could work across party lines to improve the process. It was happening anyway and I am proud we've managed to improve it."
He feared not acknowledging the support for the flag would alienate people from the political process.
"We know it was young people who engaged most with the Red Peak flag. My big fear was had this Parliament ignored that massive groundswell, we simply would have disillusioned those tens of thousands of Kiwis who may be for the first time engaged with a political issue."
NZ First MP Denis O'Rourke compared the Red Peak flag to a "corporate logo".
"What we have already said in New Zealand First is that this process is a sell-out for the New Zealand flag.
Many people had criticised the cost of the $26 million price tag of the referendum and Mr O'Rourke says more has been wasted on this amendment.
"The fact is this Bill actually wastes $8 million of process that's already taken place and never has there been such an utter and complete waste of money or indeed time of this House because of the introduction of this particular unnecessary Bill."
Support for the alternative flag grew after the independent Flag Consideration Panel's shortlist of four flags met with an underwhelming response.
Mr Key said people were slow to engage with flag change process because the panel was responsible for deciding the four flags.
"Ultimately the public did speak. All we tried to do was be relatively pragmatic about that."
He said he and the Government would be criticised in any situation.
But Red Peak's inclusion might not make much difference to changing the flag, with the latest 3 News/Reid Research poll showing only 25 percent of people want to change the flag, while 69 percent would opt to keep the current one.
However, the poll was taken before the law change was made.
Mr Dustin says he was pleased to see Parliament "come together and respond to a genuine spontaneous groundswell".
"I am delighted that there will now be some real choice in the referendum. The greatest outcome for me personally is how New Zealand has become engaged with the process of this incredibly important decision," he says.
A petition with 50,000 signatures to include Red Peak was presented to Parliament and received by the Greens, Labour and the Maori Party.