Upper house MP Louise Kingston has accused WA Opposition Leader Shane Love of "relentless bullying and harassment" in parliament after resigning from the National Party.

The Legislative Council was told the news just after 5pm today before the house rose for the winter recess.

Speaking through tears, an emotional Ms Kingston said it gave her "no pleasure to have to say what I am going to have to say today".

"I have been the victim of relentless bullying and harassment by the leader of the Nationals," the member for the South West region told Parliament.

"I raised it with others and addressed it directly with him.

"I believed the situation had been resolved, so therefore did not pursue a formal process through the party.

"I believed everyone had learnt from the situation and used the opportunity to grow and build better relationships.

"Unfortunately I have now learnt a hard lesson."

Resignation 'disappointing'

Ms Kingston said she had no issues with any other MP, all of whom had been supportive.

Shortly after her speech, Mr Love released a statement saying he had received Ms Kingston's resignation.

"While it is disappointing Ms Kingston no longer wishes to be part of our Nationals WA team, we thank Ms Kingston for her service to our party," it read.

Earlier this month she was preselected in fifth place on the party's upper house ticket, generally considered an unwinnable spot.

The Albany-based business owner told parliament she had been "punished by an underhanded campaign and the party decided I was not worthy to be a part of their team going forward".

"People come and go but in more recent times many talented people have left and I now join those people."

It means four of the Nationals six MPs won't be contesting the next election.

Long-term MPs Mia Davies and Martin Aldridge have previously announced their resignations.

Colin de Grussa earlier this month made a last-minute decision to not seek pre-selection in the upper house.

Mr Love and his deputy, Peter Rundle, are both re-contesting.

'Agonising decision'

"I sincerely hope that this leads to further changes in the party to address the issues I have raised," Ms Kingston said.

"It is too late for me but others need to be protected into the future.

"This has been an agonising decision and I desperately hope that this will effect the change needed and protect others from what I have endured.

"I want to thank those that have reached out and supported me. It means so much to know at a time like this you do still have a value."

Ms Kingston said she would now consider her future but promised to continue speaking out "for those that can't and seek solutions to help others be the best they can be".

Because Ms Kingston's comments were made in the Legislative Council she is protected by parliamentary privilege, which grants her legal protection against defamation action.

WA Nationals state director Julie Kirby told the ABC she had been caught by surprise by the allegations and "acknowledged" Ms Kingston's disappointment at the pre-selection process, noting the upcoming election is the first time the upper house will be elected from one state-wide electorate.

Ms Kirby said while no bullying complaint had been lodged, "we will take any allegations very seriously".

She said her next step would be speaking to Ms Kingston about how she can be supported before deciding what other action to take.

Nationals WA president Julie Freeman said the party acknowledged "Louise was disappointed with the outcome of the preselection process".

"Our Legislative Council preselection is always robustly contested, and this was no different," Ms Freeman said.

"The candidates are selected by delegates who are democratically elected by our grassroots membership."