An investigation into sexual harassment allegations levelled at Barnaby Joyce has been unable to make a finding due to "insufficient evidence".

West Australian farmer Catherine Marriott made the complaint in February, leading Mr Joyce to resign as deputy prime minister and leader of the Nationals.

But she was told on Thursday that after an eight-month investigation the NSW branch of the Nationals had been unable to make a determination about the case.

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"This is despite the investigation finding I was "forthright, believable, open and genuinely upset" by the incident," Ms Marriott said in a statement on Friday.

An "extremely disappointed" Ms Marriott said she was not surprised at the result because the party had no plans for how to handle a sexual harassment complaint against an MP.

She made three trips to the east coast at her own expense to meet with the party, and her name and confidential complaint were leaked to the media.

"While it has come at enormous personal expense, I was not prepared to walk past this kind of behaviour any longer," Ms Marriott said.

"I am pleased I stood up for what I believed was right."

Mr Joyce stood down in February after Ms Marriott's allegations were raised, describing them as the last straw.

He was already under pressure to resign following his affair with ex-staffer Vikki Campion, who he now has a child with.

AAP

Mr Joyce later said he knew he would have to resign once Ms Campion got pregnant, but he hung on out of spite as the government bled support.

Ms Marriott said the only positive to come from "this harrowing experience" has been the Nationals' development of a much-improved policy for dealing with complaints.

The Nationals released a statement on Friday revealing the investigation was complete, but the result would stay confidential.