Prime Minister John Key has rejected claims by Andrew Little that he was "a liar" over when he was first told about the personal issues relating to former National MP Mike Sabin.

"It's a slightly disappointing path he is going down that other leaders did," Mr Key said, in response to claims Mr Key had lied.

"In the end I knew on December 1. His argument is that as Prime Minister I must know everything. But if that's a logical argument, why would his office ring my office to give me a heads up."

Mr Key says his chief of staff, Wayne Eagleson, found out on November 25 that Mr Sabin was being investigated by police. He went on to gather further information and Mr Key said he had been comfortable with the process Mr Eagleson had gone through.

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Mr Key said that given the information he had at the time, he believed there was no need to stand down Mr Sabin as chairman of the law and order select committee.

Mr Sabin resigned on Friday last week and a byelection will be held on March 28.

Mr Little said he made his claim on the basis of his belief that the police would have told the Police Minister early on that they were investigating an MP and that the Police Minister would have immediately told the Prime Minister.