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Mr Williamson told a senior police officer that the businessman, Donghua Liu, who was facing domestic assault charges, was "investing a lot of money in New Zealand" and urged police to be on "solid ground", according to internal police emails.
Mr Williamson resigned as a Government Minister this morning following Herald revelations that he made the phone call, said that he "in no way was he looking to interfere" with the criminal case against Liu but just wanted to "make sure somebody had reviewed the matter to ensure we were on solid ground as Mr Liu is investing a lot of money in New Zealand", according to Inspector Gary Davey.
At a press conference in Auckland today, Mr Key said it was a long established principle that government ministers and MPs did not get involved with police prosecutions.
"I think Mr Williamson, by making that phone call, crossed the line.
"There's no grey in this, in the end there's a line. The line says that ministers do not involve themselves in police prosecutions, because constabulary independence runs at the heart of the New Zealand judicial system,'' Mr Key said.
"Ministers cannot, in my opinion, make phone calls, when there's an ongoing prosecution, whatever the motivations.
"The minute he made the phone call, in my view, he crossed the line.''
Mr Key said he was unaware of Mr Williamson's motivations for making the call.
"I'm sure he's got good reasons for why he made the call.''
Emails sent by Mr Williamson to police made it clear that he was not trying to interfere with the police prosecution, Mr Key said.
However, a Government minister independently prompting a police review was unacceptable, he said.
"Making that phone call, and the police therefore undertaking a review, is wrong.
"In the end, he's paid a very heavy price, he's lost his ministerial warrant.
"I'm sure he's got honest motivations for making the phone call ... nevertheless he's made the phone call.''
Mr Key said Mr Williamson offered him an explanation for his reasons behind making the phone call to police, but the reasons were irrelevant.
"He made a phone call, and as a result of making that phone call, the police at a senior level undertook a review.
"Police should not be undertaking a review because of a minister making a phone call. I'm not blaming the police, they are responding to a minister making a call. That minister shouldn't have made that call, it's as simple as that, it's completely black and white.
"He made a phone call while there was an ongoing police investigation, that is just a no no in everybody's book," Mr Key said.
Mr Key said he spoke to Mr Williamson last night and his resignation as minister was the appropriate response in wake of his actions.
The affair was incomparable to Judith Collins' Oravida fiasco, Mr Key said.
"On every single case, I look at them on a case by case basis, I assess them, I get proper, professional and advice from the cabinet office and if I think there's a case to answer and a legitimate case, I act.
"I've acted with other ministers and I've acted in this situation.
"What New Zealanders will expect of me as Prime Minster is to uphold the values that I believe in, one of those values is constabulary independence. When someone breaches that, then I act."
Mr Key said every situation was different and had to be assessed on its merits.
"I'm not going to go through the other one, except to say that I went through the same process."
Speaking to media to media at a press conference at his electoral office this afternoon, Mr Williamson said he would contest his position as MP for Pakuranga at this year's election.
He said he was not asking for special treatment for a rich Chinese businessman and financial backer, and that on numerous occasions he had called the police for his constituents and other members of the public.
Mr Williamson said he made five or six calls to police each year on behalf of people who approached him.
In Mr Liu's case he said: "There was no intention to do anything about screwing the outcome, but just to work out the focus of it.
"When I hung up, I literally did not see that that was anything other than what a member of Parliament would normally do on behalf of somebody who had asked.
"In 26 years as an MP when I've hung up the phone from a call to ACC or the police or the health board advocating on behalf of somebody, I've always thought that was my job and I wasn't crossing a line.
"However it has become clear that the police believe that it does cross a line, the Prime Minister thinks that it was inappropriate for me to have made the call."
He said he was told of the December incident by a friend of Liu and was "shocked" due to Liu's clean record required for citizenship here.
He said he was told by Mr Liu's interpreter there was confusion over the incident.
"I said I would find out from the police what the status of all this is and has it come to an end."
Mr Williamson said he was not "friends" with Liu.
"It is pretty hard to have a friend that you cannot speak a word of their language and they of yours.
"I don't socialise with him."
Mr Williamson said he was feeling "pretty much shattered and pretty gutted".
"My family's in a bit of tatters.
"There is clearly a perception that a member of parliament should not call the police at all about a case and I will make sure I will never do that again.
"We're here because of me having resigned. I've admitted I made an error of judgement."
Mr Williamson said he had brought up Liu's financial holdings "background" for the police.
"I said he was a very large investor in the building and construction sector ... as a bit of background to who he was.
"I have a perception that we economically need a huge level of investment above what we have now."
He also said he took domestic violence seriously.
"I know about domestic violence. I'm a big supporter of Women's Refuge here and I've raised money for them here."
Labour leader David Cunliffe told reporters that Mr Williamson's case was "yet another example of the decline in standards of this National Government".
Mr Williamson followed other ministers Richard Worth, Pansy, Wong, Nick Smith, John Banks, Peter Dunne, and Judith Collins in breaching the standards that the public would expect of ministers, the Labour leader said.
He added: "Any minister who interfered in police matters, particularly for a donor, needs to be instantly gone."
Asked whether Mr Williamson should leave Parliament, Mr Cunliffe said it was a legal matter and not for him to comment on.
Green Party leader Russel Norman said questions remained about Mr Williamson's dealings with police.
He said the minister's resignation did not dispel the larger issue of ministerial impropriety on behalf of National Party donors.
"It is very important the public knows the nature of Maurice Williamson's contact with the police.
"Was he seeking to get a National Party donor off a domestic violence charge? Are there other legal issues that sort of intervention triggers?"
He said it was "particularly distressing" that the intervention related to a case of domestic violence.
"All MPs should be supporting the elimination of domestic violence and have a zero tolerance policy towards it."
Dr Norman questioned when the Prime Minister was first made aware of Mr Williamson's actions.
"Was he simply hoping that information wouldn't come out and has he been involved in covering it up?"
New Zealand First leader Winston Peter said Mr Williamson's offence was far less than Justice Minister Judith Collins' dealings with her husband's company Oravida.
He said Mr Williamson's resignation showed a double standard in National's response to conflicts of interest.
"Clearly the Prime Minister is attempting to show that he does have some standards but ironically Mr Williamson's case, as with others, proves just how duplicitous those standards are."
"If Mr Williamson has offended, and he has, then why will the Prime Minister not act in the case of Judith Collins?"