"While an incumbent Government has never won a byelection in a seat it does not already hold, the National family are united behind Matthew as he accepts the challenge to wrest the seat which Labour has held since 1922."
Lianne Dalziel had a majority of 5334 in 2011 but National polled 4152 higher than Labour in the party vote.
Six Labour nominees will seek selection in a contest on September 21.
The nominees are James Caygill, a manager with Ngai Tahu; Karen Hayes, a nurse and community worker; Christina Laalaai-Tausa, a part-time political science lecturer; Tina Lomax, principal of Kingslea School; Deon Swiggs, a community advocate; and Poto Williams, regional manager of St John of God Hauora Trust.