"As the discussion went on she withdraw her nomination."
Despite the party losing all but one of its sitting MPs, Dr Brash said the list had candidates with "good experience".
"I think we've got a good line up with very good candidates."
Dr Brash would not confirm Ms Isaacs was the mystery candidate, only saying it was a woman with a "high profile".
He hoped to confirm the name in the next three or four weeks depending on her availability.
Dr Brash said he had not spoken to Ms Calvert and would not say if he would have liked her to remain an MP.
It will be a bitter pill for Ms Calvert to swallow, given that her swing away from supporting Mr Hide as leader in favour of Dr Brash was what drew the curtain on Mr Hide's leadership - although he resigned before a caucus vote could be held.
Ms Calvert had wanted to stand again, but she was told on Saturday the board was not going to rank her highly, meaning she had virtually no chance of returning to Parliament as an MP. She then withdrew her candidacy.
"I think I was just not the fit they wanted for the team. I don't like to think of myself as being dumped," Ms Calvert said.
"If it was the choice of the board to have a team I could be part of, that would have been a good thought from my point of view, and if they don't, that's fine, too. I haven't been feeling that I've been struggling ... But I don't have either any regrets or any deep feelings that I should've done a lot better than I did ... I hope I've contributed something."
She had made no decisions on her future, but wanted to remain a member of the party.
Parliamentary leader John Boscawen said he would have supported Ms Calvert standing again.
"But the board clearly indicated other people they wanted in the top 10," he said.
Ms Calvert once referred to the airspace above the beach while debating the foreshore and seabed legislation, and during a 48-second speech suggested putting a ping-pong ball in a Labour MP's mouth.
She angered animal welfare groups by saying she did not care about the plight of battery hens.
"I don't care if they display the characteristics of chopping their heads off and running around the garden without their heads either. We care about people ahead of silly little chickens," she said.
Her departure will leave Mr Boscawen as the only current MP standing again, as Sir Roger Douglas and Heather Roy are leaving at the end of the term.
The list features two young candidates: David Seymour, 28, at number six, and lawyer Stephen Whittington, 25, at number eight.
Mr Seymour has worked for a right-wing think-tank in Canada and has floated the idea of community juries to decide the level of support for beneficiaries. He also referred to government workers as "little Hitlers", a term that Dr Brash has used repeatedly this year.
Party president Chris Simmons is number seven.
Act has five MPs but on present polling the party would only gain three seats.
ON THE LIST
1. Don Brash
2. John Boscawen
3. Catherine Isaac (subject to availability)
4. Don Nicholson
5. John Banks
6. David Seymour
7. Chris Simmons
8. Stephen Whittington
9. Kath McCabe
10. Robyn Stent
