Labour leader Andrew Little says his caucus still retains "a good mix" of MPs despite another departure from the business-friendly, right-leaning side of the party.
Clayton Cosgrove, a fifth-term list MP, confirmed today that he would not stand at next year's election.
He is the latest of Labour's right-leaning MPs to leave the caucus, following in the footsteps of Shane Jones, who left in 2014 to work as an ambassador for Pacific economic development, and Phil Goff, who is leaving to contest the Auckland mayoralty.
That effectively leaves just David Shearer and Damien O'Connor on the more business-focused side of the parliamentary party.
Mr Little downplayed the changing makeup of his party, saying his caucus still had a broad range of MPs and was recruiting from across the political spectrum.
"We remain a good mix. I look at potentially new arrivals lined up for next year's election and we've got a good mix there as well.
"People expect us to be a broad-based party and we will continue to be that. That will be reflected in our caucus."
Mr Cosgrove said he was leaving to pursue new challenges, most likely in the corporate world. He had "huge admiration" for Mr Little, who was "a good mate" and had "set Labour on the right course".
He has been considering resignation from Parliament for some time. It is understood Labour's decision to oppose the Trans-Pacific Partnership (TPP) was a factor in his decision to move on. Labour's position on the major trade deal also rankled with Mr Goff and Mr Shearer, who broke ranks in January and said they supported the trade deal.
Mr Little said Mr Cosgrove had told him he was ready for a change of career, and that "there were a lot of factors" behind his decision to step down.
Mr Cosgrove would not rule out leaving Parliament ahead of the election, though he does not have another job lined up yet.
The next person on the Labour Party list is former MP and former party president Maryan Street, who is based in Nelson. She did not return calls, but said on Facebook that she had not made a decision on whether she would return to Parliament.
Ms Street -- who is leading a campaign for the legalisation of assisted dying in New Zealand -- is from the left of the party and played a leading role in removing Mr Shearer as Labour leader in 2013.
Below her on the list is another former MP, Moana Mackey, followed by Indian-born, Auckland-based Priyanca Radhakrishnan.
Mr Cosgrove, 46, entered Parliament in the Waimakariri seat in 1999. His majority in the seat was gradually chipped away as boundary changes turned his electorate from a mostly urban seat to a mostly rural one. He lost the seat in 2011 and failed to win it back in 2014.
A "tribal" member of the Labour Party since age 14, Mr Cosgrove was later a protege of former Prime Minister Mike Moore, running his campaigns in the 1990 and 1993 elections.
In Parliament, he held several ministerial portfolios including building and construction, associate finance, immigration, small business, responsibility for the Rugby World Cup, and associate justice.
He said his biggest achievements were reforms of the real estate and property sector, improving quality in the construction sector, and leading the development of the business case for the redevelopment of Eden Park for the 2011 Rugby World Cup.