Don Brash. Pic: NZPA

Wellington, June 14 NZPA - Former National Party MP Richard Worth's resignation from Parliament was "inevitable", says former party leader Don Brash.

Dr Worth resigned late on Friday after earlier being asked to resign as a minister when it emerged that he was under police investigation over allegations of a sexual nature involving a Korean business woman.

Dr Brash said "sadly... it was inevitable" he would resign as an MP altogether.

"I was leader for a time when he was a member of the Caucus and he was an active member of the Caucus and made a contribution to it," he said on TV1's Q+A show.

Political commentator Therese Arseneau said the public had already forgotten about the situation but it may haunt the Government in the future.

"These sorts of things do add up over time." she said.

"I think in the longer term a government starts to look error prone, or it starts to look more vulnerable."

In a statement announcing his resignation, Dr Worth denied committing any criminal offence and said he made his decision for the good of himself and the National Party.

Prime Minister John Key said on Friday Dr Worth's resignation was "a sensible course of action in the circumstances".

"The decision gives the country an opportunity to move forward and focus on the issues that matter."

Police have contacted Dr Worth over the allegations but it was not known if he had been interviewed by them yet.

He said previously he was cooperating with their investigation.

A second allegation was also made against Dr Worth by a member of the Labour Party.

The woman involved, Neelam Choudary, alleged that between November and February, Dr Worth sent dozens of text messages, some sexually explicit, and called her numerous times.

In a public statement she also said Dr Worth offered her jobs in his ministerial capacity.

Labour leader Phil Goff said Dr Worth's resignation was expected.

When a leader loses confidence in a list MP, "as John Key obviously had", there is no other choice but to resign, Mr Goff said.