Liberal MP Ben Morton has spent his summer break rushing to confirm he is not a dual citizen, after he found out his grandfather was born in Ireland.

Key points: Over Christmas Liberal MP Ben Morton discovered an old passport listing his grandfather's birthplace as Dublin

Over Christmas Liberal MP Ben Morton discovered an old passport listing his grandfather's birthplace as Dublin The Irish embassy has confirmed he is not an Irish citizen

The Irish embassy has confirmed he is not an Irish citizen Constitutional issues still cloud Parliament despite multiple High Court cases

The discovery, made while going through old family records over Christmas, contradicts his declaration from 2017 that both maternal grandparents were born in the United Kingdom.

A letter from the Irish embassy received this week confirmed that Mr Morton is not an Irish citizen, and Mr Morton has said he has "never been an Irish or British citizen, only ever Australian".

However the new development highlights the uncertainty that still surrounds the Parliament and section 44 of the constitution, which disqualifies dual nationals from being MPs.

Mr Morton declared in 2017 that he had conducted his own "extensive investigations" about his qualifications prior to nominating and received a letter from the British Home Office confirming he was not a British citizen.

But he has now amended his entry on the citizenship register after the discovery of papers as his father was moving house.

"While sorting through these papers over the Christmas break we found a photocopy of a British passport belonging to my maternal grandfather," he wrote.

"His nationality was confirmed as 'British citizen'. His place of birth however was listed as Dublin."

A letter from the Irish embassy, dated January 7, stated a search of the Irish foreign births register had been conducted and that Mr Morton was not registered.

Mr Morton wrote that his earlier declaration that his maternal grandfather was born in the UK was based on advice from his grandmother's relatives as his mother died in 2015 and his grandfather died in 1988. His grandmother has also passed away.

Citizenship uncertainty remains

The register was set up by Malcolm Turnbull following the citizenship crisis of 2017.

It requires MPs to amend the register within 21 days if they discover new information, but only requires MPs to make declarations to the best of their knowledge.

Tasmanian independent MP Andrew Wilkie is critical of the approach and has called for an independent audit.

"I've argued from the start that the only way to bring genuine clarity to the citizenship status of federal politicians is for an independent auditor to examine everyone's background," he said.

"The revelation that Ben Morton has been entitled to Irish citizenship all along is a striking demonstration of the continuing need for such an audit."

Anne Twomey, professor of constitutional law at the University of Sydney, said the new system had helped because MPs must now "apply their minds to the issues and to update the register when they find out that their position has changed".

But she said no system could remove all uncertainty about section 44.

"There will always be circumstances where the facts are unclear, the legal situation uncertain or the position has changed over time, for example when another country changes its law regarding citizenship and does so with retrospective effect," Professor Twomey said.

George Williams, a dean of law at the University of New South Wales, has called for a referendum to alter section 44.

Professor Twomey predicted any referendum would be unlikely to succeed, but said there would be merit in clarifying the more uncertain aspects of section 44.

Mr Wilkie believes more MPs have questions to answer.

"I am in no doubt that there are other MPs with questions over their citizenship, and in particular those who are fudging their understanding of the word 'entitled' or refusing to post relevant documentary evidence on the parliamentary register."