Prime Minister John Key says that Rodney Hide would keep his ministerial portfolios if Don Brash rolled Mr Hide as leader of the Act Party.

This morning Mr Key, who is in London visiting the Queen and British Prime Minister David Cameron, said he did not want to speculate on what would happen if Dr Brash ousted Mr Hide in a leadership challenge.

But he said Mr Hide would be able to remain a Minister of the Crown.

"Well, I can't see why that would change," Mr Key said.

He did not put much stock into poll results that show that Mr Hide's grip on the Epsom seat is in jeopardy.

"I wouldn't want to read too much into that. I've seen that polling in the paper a month or so ago. It's a wee way out til [the general election on] November 26."

Last week Mr Key said National would primarily target the party vote in Epsom, potentially giving Mr Hide an easier run, but he said yesterday the strategy could change.

"That is not locked in stone ... We haven't had formalised discussions with the Act Party," he told Newstalk ZB.

Dr Brash has said if his leadership bid is successful, he would like former Auckland Mayor John Banks, who is also a National Party members, to stand for Act in Epsom.

Mr Key said he did not know if Mr Banks, who has not ruled in or out a return to national politics, wanted to stand for National again.

"I don't think that's likely that he would want to. I haven't had any indications from him that he does. In fact, I haven't had any indication from him that he seriously wanted to return to parliament."

What happened to the Act Party was up to its members, he said.

"I wouldn't want to second guess what they're going to do. At the end of the day, I'm the leader of the National Party and we work with our coalition partners, but who leads Act and who they might stand as candidates in the general election, that's a matter for them."

He said he was not worried that Act seemed unstable.

"There are some things I can control and some things I can't. In the end, the destiny of Act is in the hands of its own members and its own caucus."