Police have defended a review of the criminal prosecution against a wealthy National Party donor after being contacted by former Minister Maurice Williamson.

Mr Williamson today resigned his ministerial portfolios after the Herald revealed he phoned a senior police officer about the criminal charges that Liu was facing. Prime Minister John Key said today Mr Williamson "crossed the line", despite assuring him that he did not intend to influence the prosecution.

Mr Williamson resigned his ministerial portfolios after the Herald revealed he called Superintendent John Tims — the district commander for Counties Manukau — in January about a family violence incident involving Donghua Liu in December.

The inquiry was referred to his Auckland counterpart, Superintendent Mike Clement, who tasked a senior officer to review the case and respond to Mr Williamson, who is still the MP for Pakuranga.

Internal police emails show Inspector Gary Davey told the two district commanders that he phoned Mr Williamson on January 28.


"He started by saying that in no way was he looking to interfere with the process, he 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."

The criminal case was reviewed by the senior sergeant in charge of family violence cases, as well as the police prosecution team, and Mr Davey said he told Mr Williamson the police would carry on with the prosecution.

"I also explained the wider responsibility of police to pursue these matters [redacted] ... The conversation was polite and professional on both sides and he appeared to be accepting of the police position."

Asked whether the police should have reviewed the Liu case at all, a spokesman for Police National Headquarters said "on occasions, police will review matters for a variety of reasons", including queries by the media, lawyers and the public.

"While not all such reviews result in a direct response to the initial inquirer, some do, as was the situation [here]," the spokesman said.

He confirmed that PNHQ was not alerted to Mr Williamson's phone call until an Official Information Act request was lodged by the Herald last month.