Liberal MP John Alexander may have made a "rash decision" to resign and spark a by-election as UK officials were unable to confirm he held British citizenship.
Mr Alexander has received advice from the UK Home Office that he was no longer entitled to become a dual citizen, but it remains unclear whether he ever held British citizenship.
Constitutional law expert George Williams says it may have been unwise for Mr Alexander to resign and spark the Bennelong by-election until the High Court clarified his case.
"If he hasn't had confirmation that he's a British citizen, it would seem like a somewhat rash decision," the UNSW Dean of Law told AAP on Friday.
"If it's not clear, he would have been rightly advised not to resign because he may well survive a High Court challenge.
"Everything I have seen in regard to Alexander would have led me to believe that he wouldn't have resigned unless it was clear that he was a British citizen."
However, University of Sydney Professor of Constitutional Law Anne Twomey disagrees.
Prof Twomey says the evidence suggests Mr Alexander was "extremely likely" to have been a dual citizen even if it can't be proven "100 per cent one way or another".
"He took the appropriate step of resigning and calling a by-election," Prof Twomey told AAP.
Mr Alexander's father came to Australia as a four-year-old in 1911 - 40 years before his son was born in Sydney.
"Unless his father had renounced his citizenship then Alexander was entitled to UK citizenship," the Ms Twomey said.
"Simply because no one can utterly and completely prove whether he did or he didn't, that means there's still some doubt.
"It would be far better to renounce any possible citizenship ... rather than going on and waiting to be referred to the High Court."
A statement from the Liberal Party suggested it was unclear whether Mr Alexander was entitled to citizenship.
"He has received confirmation from the UK Home Office that he has successfully renounced the UK citizenship that he may have been eligible for through descent," the statement read.
Mr Alexander said he stood by his decision to resign because he believed at the time he "most likely" held dual citizenship by descent - which would disqualify him under section 44 of the constitution.
The seven other MPs forced to leave parliament were all confirmed as dual citizens.
"Today started off with some great news ... I had official notification from the (UK) Home Office that I am indeed something that I've always believed I am - Australian and solely Australian," he told reporters in Sydney on Friday.
Mr Alexander is now clear to defend his seat in the December 16 by-election against Labor's former NSW premier Kristina Keneally.