In the wake of Rodney Hide's resignation, the Act party has unanimously voted to accept Don Brash as leader.
Dr Brash told Fairfax the party's board met today to vote on the matter and ratify both his membership and leadership.
The caucus will meet on Tuesday.
Dr Brash ousted Hide in a coup which culminated on Thursday, when Hide announced he would endorse Dr Brash to take his place.
Dr Brash said the main issues he was promoting aligned with the party's priorities, including government spending and borrowing.
A former leader of the National Party, he has long been dissatisfied with what he says is the Government's poor performance.
He has become increasingly frustrated as chair of the 2025 Taskforce, whose recommendations Prime Minister John Key has quickly and repeatedly dismissed as too extreme.
If Dr Brash can lead ACT back into Parliament in the November election, the party will sign a support agreement with National as it did after the 2008 election.
That would mean a ministerial role for Dr Brash and he has set his sights on finance.
But Mr Key has said it is "highly unlikely" Dr Brash would be given that position.
Dr Brash said yesterday it was too early to judge that.
"I think it's too early to say what the prime minister would rule in or out," he said on Radio New Zealand.
"That will depend on how strong the two parties are ... there are too many imponderables to be able to answer that."
Mr Key said the changeover did not mean much for the current government.
"They might have a new leader but the direction and views haven't changed, and from our perspective we will work with the ACT Party, hopefully constructively, as we have in the past," he said.
Critics of Dr Brash are warning that his return to Parliament could result in extreme right-wing policies but Mr Key said he intended to run a moderate, centre-right government.
"I don't think we vehemently disagree with ACT about the direction New Zealand should take," he said.
"Where we have significant disagreement is about the pace and extremes of the changes."
Labour's finance spokesman, David Cunliffe, said Finance Minister Bill English's position was under threat.
"Don Brash clearly has his sights set on becoming finance minister after November 26 in the unhappy event for New Zealand that an extreme right coalition is elected," Mr Cunliffe said.
"To become finance minister he would have to roll Bill English. He's rolled him before to become National leader, and he has just rolled Rodney Hide, so Bill English would be well aware how serious the threat is."