ACT will go into the election campaign as the only parliamentary party without a sitting MP seeking re-election.
The party's parliamentary leader, John Boscawen, announced on Saturday he will retire from politics at the general election on 26 November.
Mr Boscawen told Morning Report on Monday his retirement should not be seen as him abandoning ship and his departure has nothing to do with Dr Brash or how the party is faring in the polls.
Mr Boscawen said politics is very time-consuming and he wants to spend more time with his family.
His departure marks a full changing of the guard since Dr Brash became leader earlier this year.
Former ACT leader Rodney Hide, Sir Roger Douglas, Heather Roy and Hilary Calvert are also giving up politics.