The defeated West Coast-Tasman MP Damien O'Connor will return to Parliament following the departure of Michael Cullen.
Dr Cullen is leaving Parliament at the end of the month to become the deputy chairman of New Zealand Post.
The Labour Party leader, Phil Goff, confirmed Mr O'Connor will take up the empty seat, as he is next on the party list. His first day back will be when the House sits on 5 May.
Mr O'Connor had been the West Coast-Tasman electorate MP for 15 years, but lost the seat to National's Chris Auchinvole in last year's election.