Lianne Dalziel's looming resignation to contest the Christchurch mayoralty has sparked speculation Labour will stand list MP Clayton Cosgrove in the byelection for the seat, opening the way for Northlander Kelvin Davis to return to Parliament.
Ms Dalziel confirmed she would stand against Mayor Bob Parker in the local body elections in October and would resign as an MP before then.
She said she was likely to time her resignation so a byelection would not clash with the local body elections - something she said would be unfair to candidates in both.
That means a mid to late November byelection is likely.
Ms Dalziel would not say whether a new person or a current MP should replace her but expected the electorate to attract a large number of candidates. Some in Labour believed Mr Cosgrove, who lost the Waimakariri seat to National's Kate Wilkinson in 2012, should stand in Christchurch East instead. Other possibilities mentioned within Labour include Problem Gambling Foundation adviser Tony Milne and former Labour MP David Caygill's son, James Caygill.
Mr Caygill is a consultant to Ngai Tahu and has worked as an adviser to Helen Clark.
Mr Cosgrove is the only list MP Labour has in Christchurch and if he won the seat it would open the way for the highly regarded educator Kelvin Davis to return.
Mr Cosgrove would not say whether he intended to stand. "It's very humbling to be mentioned in dispatches, but it is all speculation."
Prime Minister John Key said National was also likely to stand in the byelection.
National will have to find a new candidate - in 2011 it stood Aaron Gilmore, but he resigned his list seat in the House recently.