Rachel Skelton has been appointed acting principal of the Selwyn school while a recruitment process is under way to find a permanent replacement for former principal Steve Saville.
Mr Saville, who was the school's founding principal, announced his resignation last month. He is moving to Auckland to take up an education consultancy role.
Ms Skelton is a member of the school's senior learning team.
Said board of trustees chairwoman Lynley Shaw: "The board expects to recruit and appoint the new principal during term one with the intent that the successful candidate will commence their position early term two."
She also announced the stage two extension of the school will begin in the second half of next year.
"This is with a view to having the new additional building open by 2022.
"This will allow for the college roll growth to increase from 1100 to a maximum of 1800 learners.
Ms Shaw said some areas within the existing building will also be altered to allow for future growth in some of the specialised areas.
