The Labour Party expelled MP Chris Carter late last night after a meeting that lasted more than six hours.

Party president Andrew Little said Mr Carter's actions on July 29, when he sent anonymous letters to journalists in Parliament's Press Gallery undermining leader Phil Goff, were likely to foment internal discontent and encourage external ridicule.

"They were deceptive, they were clearly calculated to cause damage to the caucus and foment discontent and disharmony in the caucus and cause damage to the party's reputation," Mr Little said.

"At no time has there been an acceptance of the gravity of [the actions] or a display of contrition."

Mr Carter, who has the right under the party's constitution to appeal against his expulsion, made his case to the party's council for four hours, including answering questions.

He then left the party's headquarters in Willis St, Wellington, at 9.30pm out the back door, avoiding waiting reporters.

The council debated until close to midnight and all members supported expulsion.

Mr Little said: "Had the author of his letter to the gallery not been revealed, it would have caused considerable harm to that caucus and the way it operates, and there's no question that the publicity in the days that followed caused damage to the party."

He said that in other leadership challenges, there had been direct contact with the leader, not a "furtive, sneaky letter".

"It has been an extraordinarily difficult decision because this has been an MP, a member of the party who has given extraordinary service, risen to very high ranks in Cabinet, has been an outstanding minister for Labour in government, and that partly goes to show why people couldn't understand how this could have happened."

Mr Little said: "What we were looking for as an indication that he understood the impact of what he had done, that
he regretted it and was contrite about it, and that he understood
that if he was serious about his affections for the party and his support of it, [he] would not make comments that would undermine the
leadership of the party and the party itself. But that was not evident
tonight."

Mr Carter was told of the decision late last night.

Mr Little said the council has trod carefully and its action would withstand any legal challenge over employment processes.

Mr Goff said he was pleased with the decision, which showed that Mr Carter's kind of behaviour was not acceptable.

Mr Carter was elected MP for Te Atatu in 1993, and was New Zealand's first openly gay Cabinet minister.