WA Liberal leader Mike Nahan has revealed his discovery that he is still a US citizen because of an unresolved tax dispute with the country's Internal Revenue Service (IRS), sparking renewed scrutiny of his leadership.

Dr Nahan recently returned from a period of leave, during which questions were raised about the eligibility of state MPs who travel on foreign passports or renew a foreign passport while in Parliament.

In his absence, the Opposition Leader's staff told journalists he was not a dual citizen.

But on Tuesday, Dr Nahan moved to "correct the record", explaining how he had requested proof his application to renounce his citizenship had been approved, only to discover it had not been accepted because of the tax dispute.

"As you know I was born in the United States, came to Australia 40 years ago, became an Australian citizen about 30 years ago," the former WA treasurer said.

"Then shortly after becoming elected in 2008, I started the process of renunciation of my US citizenship."

Dr Nahan said he thought he had done all that was required to renounce his US citizenship, and had not requested the paperwork until recent weeks.

"The US Government won't allow the renunciation until I resolve the tax issue," he said.

It is unclear exactly how much money the IRS is pursuing Dr Nahan for. ( Reuters: Jose Luis Gonzalez )

Nahan won't put dollar figure on dispute

Dr Nahan said the IRS claimed he owed it "a sizeable sum of money", with the dispute dating back to 2009.

"[It concerns] the treatment of what is taxable income under capital gains tax and other reasons, and also issues related to super funds," Dr Nahan said.

He was unwilling to put a dollar figure on the dispute but said it was a significant amount.

Dr Nahan said he was in no doubt — relying on the WA Solicitor-General's advice — that the dual citizenship did not put his eligibility at risk, but said he would continue to work to resolve the issue.

The eligibility issues were raised by a Murdoch University lecturer recently after the federal citizenship saga saw handfuls of MPs in hot water.

Dr Nahan said he had not travelled on his US passport or renewed it in the time he has sat in Parliament.

Dr Nahan and the Liberals celebrated a strong Darling Range by-election victory just weeks ago. ( AAP: Rebecca Le May )

Liberal colleagues express disquiet

The revelations have raised eyebrows among Dr Nahan's fellow Liberals, who heard about the problem for the first time on Tuesday.

The ABC has spoken to a number of his colleagues who privately expressed surprise and disappointment that he had not disclosed the information earlier.

The timing could not be worse, with Liberal MPs scheduled to meet today for their annual "love-in" at Joondalup.

But Dr Nahan maintained there was no need to disclose the information prior to taking on the leadership.

"No, why would I talk about my tax issues with my colleagues?" Dr Nahan said.

"Those are personal matters and they are being resolved under the law appropriately."

Fran Logan 'used UK passport'

WA's Corrective Services Minister Fran Logan, who returned from the UK to deal with last week's Greenough prison breakout, admitted he has travelled on a foreign passport.

Fran Logan says he used his British passport to beat airport queues. ( ABC News: Andrew O'Connor )

"I used my Australian passport in terms of leaving Western Australia and coming back to WA, and I used my UK passport to get past the queues and get straight into the EU," Mr Logan told Fairfax radio on Monday.

Mr Logan said advice from the state solicitor was, "that's fine".

WA Treasurer Ben Wyatt backed Mr Logan.

"I'm confident there's no eligibility issues in Fran Logan's [case] or others," he said.

"I'm aware of the advice … provided to the Government and as far as I can tell, that's the best advice I've heard. So I'll stick with that unless I'm convinced otherwise."