Former National Party leader Don Brash will seek the backing of the Act board on Saturday for an attempt to oust the leader of the party - which he has yet to even join.

It is understood he will form a new party if his bid for the Act leadership fails.

Dr Brash front-footed his bid to roll Rodney Hide yesterday, describing the Act brand as "severely damaged" and claiming that he was the best chance for the party to avoid political oblivion.

Dr Brash is likely to get the support of MPs Sir Roger Douglas and Heather Roy, but to become leader he would need also the backing of either deputy leader John Boscawen or MP Hilary Calvert.

He would also need the support of at least 8 of the 12 board members.

Yesterday, Mr Boscawen said he had "100 per cent" support for Mr Hide as leader, but would not comment when asked how he would vote in a leadership challenge.

Ms Calvert said she not only supported Mr Hide, but would also vote for him in a leadership challenge.

It is understood that party headquarters instructed Act MPs yesterday to say they were "100 per cent" behind Mr Hide.

If his bid fails, Dr Brash is understood to be interested in a new party, possibly involving former Auckland City mayor John Banks and former mayoral candidate Colin Craig.

He said he would be disappointed if Prime Minister John Key ruled out working with him, but he would consider sitting on the cross-benches.

Dr Brash's public challenge has taken board members by surprise, and he is yet to gauge support beyond talks with Mr Hide, Mr Boscawen and president Chris Simmons.

He will seek permission to address the party's board at its monthly meeting on Saturday.

His chances of success are not clear, but going public before even joining the party has rubbed some up the wrong way.

"We're a democratic party, and he should just come through the front door," said board member Geoff Russell, chairman of the Upper South branch. "He should walk before he runs."

He was yet to decide whether Dr Brash, 70, would be a good leader, but described him as "very old".

Ms Calvert said it was "a bit presumptuous for a non-party member to have too many ideas on what they might do".

Mr Simmons said he had spoken to all board members and they were "100 per cent supportive of Rodney's leadership and are quite taken back by the statements Don has made".

Mr Hide said Dr Brash raised the possibility of being leader after he was asked to join the party about a month ago, following the annual conference.

"He came back with the proposition that he be leader, and that John Banks stand in Epsom. It's very odd for him to be seeking the leadership of Act when he's a member of National."

Dr Brash said he would sort out membership issues this week, but stopped short of saying he would quit National and join Act.

Mr Banks, who lives in Epsom, said he could not rule "in or out" standing there for Act, but he supported Dr Brash's bid and said "the people of Epsom and Rodney Hide have certainly parted company".

The Act selection for Epsom has yet to be finalised, but it could be done on Saturday.

* Don Brash must become a member of the Act Party. He could apply and have his application approved by the board on Saturday. A majority of the board has to approve the application.

* Any member can challenge the leadership, which is decided by caucus. The challenger has to give seven days' notice to caucus of a leadership challenge.

* Caucus decides the leader by majority (three out of five).

* The decision needs to be ratified by two-thirds of the board (eight out of 12 board members).

* There is nothing in the Act constitution preventing the leader being outside Parliament, but there would also need to be a parliamentary leader in the House.