Tauranga MP Simon Bridges is set to expand his portfolio after Maurice Williamson resigned over allegations he interfered in a police investigation.

Prime Minister John Key this morning announced he had accepted Williamson's resignation after he was made aware the former cabinet minister contacted police about the charges facing Chinese businessman Donghua Liu.

Tauranga MP Simon Bridges will take over one of Maurice Williamson's porfolios after his resignation today.

Until a new minister is appointed, Simon will take over the Customs and Statistic portfolio, while current minister Nick Smith handles the former minister's Building and Construction portfolio, and Nathan Guy Land Information.

A new minister outside Cabinet will be appointed early next week.

Simon currently holds the Minister of Energy and Resources, Labour, and Associate Minister for Climate Change Issues portfolios. SunLive is currently waiting on comment from the Tauranga MP.

Key says under the "no surprises" policy, police have informed him that the minister contacted them about the arrest of Liu in December 2013.

'I have been made aware that Mr Williamson contacted Police some time ago regarding their investigation of Mr Donghua Liu,” says the Prime Minister.

'Mr Williamson has assured me that he did not in any way intend to influence the police investigation.

'However, Mr Williamson's decision to discuss the investigation with Police was a significant error of judgement.”

Liu was arrested following a domestic violence incident with his partner and her mother at the Boulevard Hotel on December 11.

Police confirmed the former MP contacted police in January in relation to the case and were advised that police prosecution of Liu would be proceeding.

Liu then appeared in court on April 1 where he pleaded guilty to male assaults female and assault with intent to injure. He still to be sentenced.

"I am satisfied that our staff acted with best intentions in their interactions with Mr Williamson and the prosecution guidelines were followed in the case," says Acting Deputy Commissioner Grant Nicholls.

"Given the matter is still before the courts, it would not be appropriate to comment further," Mr Nicholls said.

Fairfax Media reports Liu was granted residency in 2005 by former immigration minister Damien O'Connor against officials' advice. In 2010 he was also granted citizenship by Nathan Guy, then Immigration Minister, after Williamson lobbied ministerial colleagues.

The Prime Minister says the independence of police investigations is a fundamental part of New Zealand's legal framework and the former minister's actions are 'very unwise” as they have the potential to bring that independence into question.

Maurice has previously held the Transport, Science and Technology, Broadcasting and Communications portfolios.