UPDATED 3.47pm The National caucus is divided over whether to change the flag, but the Prime Minister says that's no problem to him.

An email has been leaked to some members of the media, showing about half of the National caucus turned up to a meeting on how to boost the campaign to change the flag.

John Key said there's always been a division of opinion within the party.

"Don't know what all this sort of nonsense is about. I mean, MPs meet all the time about issues.

"We don't have a caucus position because it's essentially from our point of view a conscience view. So some MPs have said they support change, some don't."

Labour Leader Andrew Little's not surprised National MPs are split on the issue, given polling shows most New Zealanders are opposed to a flag change.

"I imagine National MPs are getting it in the neck from constituents which is why they're getting a bit anxious-ridden about it.

"But I think the problem they've got is of course the train's left the station, the referendum's in a couple of weeks time, I'm not quite sure what they think they can do about it."