Disgraced former Minister Maurice Williamson will contest the next election despite admitting he was wrong to contact police about their investigation into a Chinese businessman whose residency he had earlier supported.

Photo: SNPA

Mr Williamson stepped down as a government minister on Thursday after the Prime Minister became aware he had contacted police over the investigation into Donghua Liu who was facing domestic violence allegations.

E-mails between three senior police officers show Mr Williamson made an initial call in January to the Commander of Counties Manukau police, Superintendent John Tims.

The District's Crime Prevention Manager Gary Davey was tasked with following the call up. He requested a review of the file and later told Mr Williamson the prosecution against Mr Liu would go ahead.

Acting Deputy Commissioner Grant Nicholls said he was satisfied staff acted with the best intentions in their interactions with Mr Williamson and that prosecution guidelines were kept to.

'Error of judgement'

After high level discussions on Thursday morning Prime Minister John Key issued a statement about 11am saying Mr Williamson would step down as a minister.

Mr Williamson is adamant he was not trying to influence police when he contacted officers over Mr Liu's case, but resigned because a perception he was doing so had been created.

"I thought that I made crystal clear I was simply enquiring as to the status of the matter," his statement said, adding police "seemed fine with the conversation."

"It is clear to me now that that was an error of judgement and regardless of what I actually said - it could be interpreted I was trying to influence the issue."

Mr Williamson said he would stay on as an MP and will contest the next election.

Photo: RNZ

Right to resign, says PM

Prime Minister John Key said as soon Mr Williamson called police about their investigation he crossed a line.

Mr Key said Mr Williamson had made a serious lapse of judgement and had done the right thing by resigning.

"It's wholly inappropriate for a minister to make a phone call to the police during an investigation. As a result of that phone call police reviewed the file. That review would not have taken place, in my opinion, if the phone call hadn't taken place."

Photo: SNPA

Mr Key said Mr Williamson had assured him he did not intend to influence the police investigation. However, he said, the action had the potential to bring the independence of the police into question.

Mr Williamson was a minister outside Cabinet and held portfolios of Building and Construction, Customs, Land Information and Statistics. Mr Key said he would appoint a new minister outside of Cabinet early next week. In the meantime, acting ministers will be appointed; Nick Smith in the Building and Construction portfolio, Nathan Guy in Land Information and Simon Bridges in Customs and Statistics.

Citizenship

Mr Liu's citizenship was approved by Nathan Guy in 2010 when he was Internal Affairs Minister, after which the businessman donated $22,000 to National. Mr Williamson supported Mr Liu's application for citizenship.

Officials from the Department of Internal Affairs had recommended against Mr Liu getting citizenship because he did not meet English-language criteria or spend enough time in New Zealand.

Mr Williamson and ACT MP John Banks, who was Auckland's Mayor in 2010, have both confirmed they advocated for Mr Liu to be granted citizenship despite officials' reservations.

Photo: TVNZ

Subsequently, Mr Liu was charged with assault and injuring with intent after an attack on his partner and his partner's mother. He pleaded guilty and is due back before the Family Violence Court in Auckland next month.