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Wellington police are investigating an incident involving National MP Richard Worth following a complaint in Auckland two weeks ago.

Detective Inspector Paul Basham said preliminary inquiries were made and the complaint has been transferred to Wellington police for further investigation.

Prime Minister John Key today revealed he previously investigated an allegation of Richard Worth "making a nuisance of himself towards women".

Mr Key said the allegation - separate to the matter being investigated by police that led to Dr Worth's resignation as a Minister today - was made in recent weeks.

"I've had someone bring an allegation to me of that nature. All I can say I treated the allegation seriously. I investigated it and I was satisfied with the answers I received," Mr Key said.

Mr Key said he received assurances the initial allegations were not correct, and he had no reason to disbelieve that.

He said the circumstances in the latest matter were different and at "first blush" warranted police investigation.

National MP Dr Worth announced his resignation as Internal Affairs Minister "for personal reasons" today.

Mr Key told a media conference this morning that it was "wholly appropriate" that Dr Worth had stepped down.

"If he hadn't resigned I would have sacked him," he said.

"His conduct does not befit a minister. I will not have him in my Cabinet. I have lost confidence in him as a minister."

Mr Key said Dr Worth had not been charged with any offence "at this point" but that police were issuing a statement relating to the MP.

The police release reads: "An allegation has been made against a sitting member of Parliament. Wellington police are conducting a preliminary investigation into this matter."

Mr Key said it was up to Dr Worth to decide how much information to give on the police investigation, which the MP had told him about late last week.

He said it was for Dr Worth to determine whether he remained as an MP.

The Prime Minister earlier said he had accepted Dr Worth's resignation last night.

"He advised me of some private matters in respect of which he felt it appropriate that he should resign as a minister."

Dr Worth said in a statement issued this morning it was "with deep regret" that he had resigned his ministerial role.

"I tendered my resignation to the Prime Minister last night and he accepted," Dr Worth said. "It has been a privilege to have been a Minister in this Government."

He said he had been granted two weeks leave of absence from Parliament.

Before the Prime Minister's media conference this morning, Labour Leader Phil Goff said Dr Worth's resignation came months too late and it was incumbent on Mr Key to explain to the public why his minister has stepped down.

"It is completely unsustainable for John Key to say that resignation from a high public office is for a secret or undisclosed reason unless there are personal family matters behind Dr Worth's agreement to resign," he said.

"The Prime Minister had ample reason to sack his Minister months ago but lacked the necessary leadership to do so."

Dr Worth was a minister outside cabinet holding the portfolios of internal affairs and land information. He was also an associate minister of justice.

Mr Key said Maurice Williamson would be covering Dr Worth's portfolios.

"Dr Worth's ministerial responsibilities will be temporarily transferred to Maurice Williamson. I will be making an announcement about permanent appointments to the portfolios in the near future."

The Governor-General has been advised of Dr Worth's decision.

Dr Worth is number 22 on National's party list. The MP faced a number of questions about his political judgement earlier this year after a trip to India during which he spoke in his ministerial capacity and promoted an aviation company in which he held shares and a directorship.

He was reprimanded by Prime Minister John Key over that trip for breaking the rules about potential conflict of interest, and Labour demanded his resignation.

He then faced questions again after it was revealed he visited a taxi driver who was allegedly attacked by passengers.

Dr Worth subsequently admitted he knew the father of one of the men accused of attacking Jarnil Sandhu in what appeared to have been a racially motivated incident in Auckland in April.

Labour Party immigration spokesman Pete Hodgson said at the time that the visit was questionable considering it appeared to have been carried out by Dr Worth in his ministerial capacity, and that he admitted knowing the father of one of those blamed for the attack.