Don Brash says it is ''logical'' John Banks will take over as ACT leader.
Brash will tender his resignation to the board today - but announced his intentions at the party's election party in Parnell last night.
In an interview with Auckland Now he said it will be a challenge to rebuild the party after it's party vote support collapsed.
But he admitted: ''The reality is we thought we'd get more than one seat.... I didn't really think the result would be as bad as this.''
ACT was saved from political oblivion by winning the Epsom seat - and as Banks will be its only MP, Brash assumes he will be leader.
''The board has to resolve that question. I assume it will be John, it's logical,'' he said.
The party will survive, he insisted.
''Let's face it, Winston Peters has just come from having had no MPs in parliament.
It wasn't a snap decision to quit, he said.
''I thought for quite some time that if I did not get into Parliament and the party got only a single MP, or worse still got none, I would take responsibility for that.
''It wasn't a difficult decision to make,'' he added. ''I think that a leader who has had a substantial loss should step down.''
But he doesn't know what he'll do next.
The party was ''knocked about'' by some pretty dismal polling in the last couple of weeks.
''People who are natural ACT supporters thought 'why waste our vote'. If we can't hold Epsom there is no point in voting for ACT.''
Most votes would have gone to National, he said.
''But I suspect some even [went] to New Zealand First."
''In any case it bled away from us and that's a great regret from my point of view.''
He had believed that ACT's policies were similar to those which National ran on in the 2005 campaign.
''I thought there would be a lot of National party people around feeling disappointed that National hadn't made more progress in those areas.''
Banks and Brash ''have had a very good relationship for more than 30 years'' and had encouraged him to stand in Epsom.
''So I've no regrets about that at all.''
Party president Chris Simmons said the party was ''absolutely delighted'' about winning Epsom.
''People had us totally gone," he said. ''We had a pretty good feeling that it was coming our way. We knew it wasn't going to be a landslide. But [we were] quietly confident.''
Simmons didn't know Brash was going to quit. ''As a board we are going to have to sit back and say 'OK John, how are we going to rebuild this.''
He said it was ''too early'' to say if Banks will be leader. ''But he is going to be our face in the House, he's going to be our key spokesman, maybe a minister.''
The board members were already looking to the future.
''I think there is a hugely bright future for the party. Yes, it's disappointing tonight because you come along and you won't to have a bunch more folks [in parliament].''
''We had our feet to the fire before, and we've come through it. We have our feet to the fire today... But we are still here, we're still alive. In 2014 we can bring a fresh team through.''
- Auckland Now
Do you have a Kiwisaver Fund?