After an exhaustive search to determine her citizenship status, Ms Kakoschke-Moore has confirmed her mother was born British in Singapore at the time it was a colony and that the senator was, herself, a citizen of Britain. MPs are not permitted to be citizens of a foreign nation under section 44 of the constitution.

Ms Kakoschke-Moore, who was born in Darwin and has not previously been connected to the citizenship crisis, said she had believed for her entire life she was not eligible because usually when a parent is born outside Britain they are unable to pass their citizenship on to their children also born outside Britain.

Her father had even made inquiries as to whether she could get a British passport when the family lived in Oman when she was 12 but he was told she was not eligible.

"Knowing that all senators would be required to provide information to the Parliament about their family history, I approached the UK Home Office last week to obtain clarification around my citizenship status to better understand why I was ineligible for British citizenship," she said.

"Their advice was extremely surprising to me. They advised that my mother was born in the former colony of Singapore before independence. This gave her citizenship of the United Kingdom and colonies under section 4 of the British Nationality Act 1948."

Following that, Ms Kakoschke-Moore said, her mother went on to obtain right of abode in Britain under the Immigration Act 1971 when it came into force by virtue of her father's birth in Britain and her birth in a Commonwealth country with a British born parent.

"When the British Nationality Act 1981 came into force on January 1, 1983, the net effect of all of this is that my mother became a British citizen under section 11.1 of the act, otherwise than by descent.

"She was therefore able to pass her citizenship on to any children born outside of the UK. I am therefore a British citizen.

"I am resigning from the Senate today and will be requesting that my matter be referred to the High Court when the Senate sits next week."

NXT founder Nick Xenophon paid tribute to his former colleague and said it was unclear who would replace her, given the recent decision by the High Court to strike out Liberal Hollie Hughes, who was set to replace former Nationals NSW senator Fiona Nash.

Tim Storer is the next candidate on the ticket for the South Australian-based party but it will be up to the High Court to decide if he fills the vacancy.

Mr Xenophon said Mr Storer may not be the replacement as he was not now a member of the party.

He predicted Ms Kakoschke-Moore's career in politics would not be over.

The eight MPs forced out for holding dual citizenship so far are Nationals former cabinet ministers Barnaby Joyce and Ms Nash, Liberal former senate president Stephen Parry and backbencher John Alexander, Greens Scott Ludlam and Larissa Waters, One Nation's Malcolm Roberts and independent senator Jacqui Lambie.

Mr Joyce and Mr Alexander are contesting byelections to win back their seats.

Another NXT parliamentarian, Rebekha Sharkie, has already faced questions about her eligibility for Parliament as she was born in England but Ms Kakoschke-Moore's resignation is the latest surprise in a saga that has dominated the Parliament for months.

Like Labor MPs Susan Lamb and Justine Keay, Ms Sharkie has insisted she is in the clear as she took reasonable steps to renounce her British citizenship before the June 9 close of nominations for the 2016 election.

The trio did not receive formal confirmation of renunciation until after the close of nominations, which could place them in breach of section 44 (i) of the constitution according to the legal opinions of experts and scholars such as professors George Williams and Anne Twomey, and former solicitor-general David Bennett, QC.

Other legal experts have expressed a contrary view and argued these MPs are likely in the clear.

The court is due to determine who will replace Mr Parry and Ms Lambie in December and Ms Kakoschke-Moore's fate may also be decided at this time.

When the Senate resumes next week, senators will have until December 1 to make disclosures about their citizenship and place them on Parliament's register of interests.

Lower house MPs will have until 8pm on Tuesday, December 5 and more MPs are, at this stage, expected to be caught up and have to go to the High Court.