Television New Zealand is investigating staff use of company resources to support political party activities.
The general manager of its Maori and Pacific Unit, Shane Taurima, resigned after it emerged he'd been involved in a Labour Party hui in January.
Documents leaked to 3News also showed the unit was used for a Labour campaign meeting in August and the company email was used to conduct Labour Party business.
TVNZ said the actions were a serious breach of its editorial guidelines and expectations of independence.
Mr Taurima unsuccessfully contested the Ikaroa-Rawhiti by-election candidacy for Labour last year and was considering seeking the candidacy for Tamaki-Makarau.
TVNZ chief executive Kevin Kenrick says after the by-election, the broadcaster sought and received assurances from Mr Taurima that he had no further political ambitions.
"And so we acted in good faith and we relied on those assurances but when we look at where we're at today, in hindsight we'd say that was a mistake."
He says clearly a line has been crossed and he makes no excuses for what's happened.
Along with an investigation into the use of resources, TVNZ will also review the editorial independence of the Maori and Pacific Unit during Mr Taurima's time as manager.
Apology from Taurima
Shane Taurima apologised on Tuesday morning and confirmed his resignation.
He said he had been a member of the Labour Party since 2013 but denied his political affiliations had any influence on editorial decisions.
In a statement Mr Taurima defended his colleagues in the unit and said they were not political activists.
He said it was unfair and unacceptable that their integrity was being questioned.
Some National MPs claim bias
Some National Party MPs and ministers claim they were subject to politically biased interviews by Shane Taurima.
Mr Taurima carried out on-air interviews for TVNZ's Q and A programme and Te Karere while he was the unit's general manager.
He stepped down as the host of Q and A after unsuccessfully running for the Ikaroa-Rawhiti by-election.
National list MP Tau Henare is scathing of Mr Taurima's conduct.
"He uses the state broadcaster to run the campaign out of the state broadcaster's office. I think it is shocking."
Justice Minister Judith Collins says Mr Taurima has made the right decision by resigning.
"If you go back and have look at some of the interviews particularly with Hekia Parata and Paula Bennett, frankly they were disgraceful."
Education Minister Hekia Parata says she felt any interviews she has had with Mr Taurima, or on Te Karere, were robust but doesn't feel she was treated unfairly.
However, Social Development Minister Paula Bennett has an entirely different view, particularly about one Q and A interview with Mr Taurima in 2012 regarding youth unemployment.
"It was one of the worst, I can say, and least informative for viewers, I've got to be honest."
Mrs Bennett says she made an informal complaint to TVNZ after the interview.
But Prime Minister John Key says he's always been treated fairly by TVNZ and its journalists.
"I think the risk for Television New Zealand is that it gives the perception of bias, which I do not believe is there. My experience is that they are extremely fair. But that's the risk and it's just not a great look."
Broadcasting Minister Craig Foss says TVNZ's internal investigation will be key in finding out if it failed to comply with the Broadcasting Act.
TVNZ head 'gutted'
Head of news and current affairs at Television New Zealand, John Gillespie, says the issue reflects badly on its editorial independence.
Mr Gillespie is 'gutted' by the situation, saying TVNZ takes its editorial independence very seriously.
"It's a cornerstone of what we do here every day, it always will be a cornerstone of what we do here every day. These people have nailed their political colours to a TVNZ mast. It is untenable."
Mr Gillespie says every staff member is aware of the editorial guidelines surrounding political parties.
A lapse of judgement'
Labour Party leader David Cunliffe has confirmed Mr Taurima is a member of the party but not a nominee for MP candidacy.
Mr Cunliffe told Morning Report he is unaware of the August meeting taking place, but there has to be clear guidelines relating to the political activities of members of the news media.
He says Mr Taurima has had a lapse of judgment.
Labour's Maori vice-president Nanaia Mahuta says Mr Taurima facilitated the January hui which was aimed at boosting Maori votes.
Ms Mahuta told Nine to Noon Mr Taurima co-ordinated the hui in his own time, and has been upfront about his political affiliations and even-handed in his professional judgement at TVNZ.