Don Brash has resigned as ACT leader saying he takes full responsibility for the party's poor performance in the election.
ACT candidate John Banks won the Epsom seat but he will be the party's sole MP as it gained just over 1% of the party vote.
Dr Brash said polls which said ACT would not hold Epsom meant people who would have supported the party felt they did not want to waste their votes.
He said he was very disappointed he will not be returning to Parliament because he is seriously worried about the state of the country.
Dr Brash said Mr Banks will play an important role in an MMP government, where the re-elected National Party needs support.
Mr Banks won Epsom by a margin of 2485 votes over National candidate Paul Goldsmith.
ACT Party president Chris Simmons said Mr Banks has campaigned hard and deserved to win Epsom.
National Party leader John Key met Mr Banks on 11 November over a cup of tea at an Auckland cafe to signal to National supporters that they should vote for Mr Banks.
Their discussion was recorded by a cameraman without their knowledge and the subsequent controversy dominated the third week of the election campaign.