Updated 4.36pm: One National MP is publicly standing by his "mate" Mike Sabin - saying they've spoken candidly about the reasons behind his resignation.

Rodney MP Mark Mitchell and Mr Sabin were in the police force together.

He says he only found out about a police investigation into Mr Sabin in early December.

They speak daily, and Mr Sabin's confided in him.

"I'm not going to discuss anything that Mike's saying. What he's says to me is said in confidence, so I'm just there as a friend to support him."

Despite that, Mark Mitchell believes Mike Sabin was right to resign.

"No, I think he did everything just right and the decision was motivated. Of course, he's got his family to consider, so I think he did the right thing."

Meanwhile there's an acknowledgement from the Prime Minister today that former MP Mike Sabin would have known he was being investigated by the police before he was told.

John Key says he was told by his chief of staff in early December, even though Labour's Andrew Little says his office told the Prime Minister's office in November.

Mr Key says he should have been told sooner.

There's an argument under the no surprises policy that we encourage people who have issues to come and tell us, but the fact of life is that he didn't and nothing is going to change that."

Labour Leader Andrew Little is adamant the Prime Minister must have known about allegations regarding former MP Mike Sabin before last December.

Mr Little says it's clear the allegations his office received, and passed on to Mr Key, were not new and had been around for some time.

"I simply do not accept the PM's word that he did not know about the allegations, before they were conveyed from my office to his office."