Officers in Auckland and Wellington are continuing to investigate a complaint against National MP Richard Worth, police said this afternoon.

The person who complained to the police about the former government minister is an Auckland businesswoman, and her allegation is of a sexual nature.

The complaint is separate to allegations Dr Worth sent inappropriate texts to another Auckland woman.

The Herald has been told the alleged incident being investigated by police took place in Wellington.

Police have said that the complaint against Dr Worth was laid in Auckland about three weeks ago and transferred to Wellington after preliminary inquiries were made.

In a statement issued today, Detective Inspector Paul Basham said the MP had "assured co-operation with the police investigation" but police had not yet decided when they would try to interview him.

"There remains a good deal of work left to complete", said Mr Basham, head of Wellington CIB.

A friend of the Korean woman complainant's last week said she told him "she had reported to the police alleging inappropriate behaviour by a minister towards her".

The Herald has agreed not to reveal the identity of the woman, the friend, or her associates.

The friend, who said he knew several MPs, including Prime Minister John Key, advised her to take the matter to the Government, and helped her to do so.

On Wednesday last week, Dr Worth announced his resignation "for personal reasons" and Mr Key made the police investigation public - saying he would have sacked Dr Worth had he not resigned first.

The police subsequently issued a statement saying they were investigating a member of Parliament after a complaint made in Auckland two weeks ago about an "alleged incident".