Ms Harvey, 52, joins Mia Davies in the Nationals and Alison Xamon in the Greens as other females leading WA's opposition parties.

"It's very significant ... I got here without being part of a quota - I'm pretty proud of that," she told reporters with a smile on Thursday.

Ms Harvey was deputy premier under Colin Barnett and was tipped to be his heir, but after the death of her husband Hal in 2014, she wanted to focus on raising their two young children.

Labor veteran Alannah MacTiernan congratulated Ms Harvey, saying it was a "great breakthrough for women in the Liberal party".

Ms Harvey said she was angry with Labor's running of the state and would focus on holding the government accountable.

"There are 99,000 West Australians unemployed," she said.

"The government's increases in household fees and charges is not only impacting on families and their ability to keep their heads above water, it's impacting on small businesses."

Ms Harvey denied her leadership would be soured by her role in the big spending Barnett government.

"This government has got that GST windfall that we didn't receive when we were in government," she said.

"I stand proud of the achievements of the Barnett government. We rebuilt the state."

In a somewhat surprising move, 65-year-old Bill Marmion was chosen as deputy despite the desire for generational change.

He beat Tony Krsticevic, 53, but the ballot numbers were not disclosed.

Mr Marmion, an engineer, former senior minister and avid hockey player, told reporters he was keen to score political goals for the opposition.

Ms Harvey said it had been a confusing week but denied the leadership ballot had been rushed.

"If you have a look at the series of the events over the last 24 months ... nothing here has been orchestrated or designed. It's been somewhat chaotic, which is a bit regrettable," she said.

Dr Nahan, the Michigan-born former state treasurer, became opposition leader after the Barnett government was thrashed at the 2017 election.

He was viewed by some as a "seat warmer" and was dogged by questions about his job, including this week.

"It was always my intention to rebuild the Liberal team, to give all leadership aspirants the opportunity to prove themselves and to then reconsider my position half way through this term, with the intention of handing over to a new team," he said.

Dr Nahan did not consult Ms Harvey before he quit, although he told the party when he was appointed he would remain in the role for the next election or as long as needed.

"I found out about that (his resignation) after he had already informed the media," Ms Harvey said.

She thanked Dr Nahan for his service and said he had been under tremendous pressure.

Ms Harvey noted the party won two by-elections in WA and the federal election during his leadership.

"He will still be an integral part of our team, but I'd like to thank him for his leadership and for keeping our team together during those two very difficult years," she said.

He will remain an MP.

Australian Associated Press