Mr Kortlang, who was an adviser in Act's election campaign this year as well as three previous ones, said it hurt the party's campaign when former leader Don Brash gave a personal endorsement for decriminalising marijuana, which Mr Banks totally rejected.
"When you've got those ideological positions, it's always going to end up in negativity - two high-profile individuals having an ideological difference on an issue," Mr Kortlang said.
"You've always got to try and manage that angst between ideological factions when you have a broad church. There was not a control mechanism around trying to control those factions. You can't differentiate the product to such an extent that it damages unity in the party."
But he was sure the party would survive, comparing the result with 2005 when survival was the goal and Act returned with two MPs.
"One MP is a platform that could eventually become five or six."
Another issue is the extent to which the party chases the law and order vote. Mr Banks was understood to be considering a ministerial role in corrections in a coalition deal with the National Party, but the Act board has wanted a focus on the economy.
A party source said they did not think people voted for Mr Banks on the basis that he could have a role in corrections.
Party co-founder Sir Roger Douglas, who has long believed in pushing the message of economic liberalism, said the party was not terminal. He said the Saturday result was disappointing and that the party had failed to capitalise on the momentum of Dr Brash taking over the leadership in April.