Long-serving Labour MP Clayton Cosgrove has announced he will not be standing at next year's election.

The list MP, who has been in Parliament for more 16 years, said today it was not a decision he had made lightly.

"I was elected to Parliament when I was 30 and now at 46, it is the right time to take the next step in my career," Mr Cosgrove said.

"Before entering politics I held senior executive positions in business both in New Zealand and Australia, and so I feel extremely fortunate to have gained so much experience in both the private and public sector.

"It has been an absolute honour serving the community as a Member of Parliament and I owe the voters and my party a huge debt of gratitude."

Mr Cosgrove said he had discussed his decision with leader Andrew Little. Under Mr Little's leadership, he was promoted to the Shadow Cabinet last year.

He said his decision to stand down was "about new challenges and opportunities".

A former businessman, he is one of Labour's right-leaning MPs. It is understood that Labour's decision to oppose the Trans Pacific Partnership was a factor in his decision to stand down.

He is the latest of several departures from Labour's right, 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.

Mr Cosgrove held the Waimakariri seat from 1999 to 2011, but lost it after boundary changes affected the demographics of his electorate.

He has been a member of the Labour Party since age 14, and was later a protégé of former Prime Minister Mike Moore, running his campaigns in the 1990 and 1993 elections.

After entering 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.