The Senate has voted to refer ACT Labor senator Katy Gallagher to the High Court over her dual British citizenship.
Labor's manager of opposition business in the Senate asked to be referred on Wednesday, saying she believed she had taken all reasonable steps to renounce British citizenship by descent from her father but delays in processing her case by the UK Home Office meant she was a dual citizen at the time of nomination for the 2016 election.
Documents provided to the Senate this week showed she was "at the date of her nomination for the 2016 election, a British citizen by descent" and that her moves to renounce in April 2016 took until August 16 to be completed by UK officials.
Senator Gallagher, the former ACT chief minister, told the Senate on Wednesday she would stand aside from her responsibilities on Labor's frontbench until her case was resolved by the High Court, sitting as the Court of Disputed Returns.
Labor and the Greens voted to support the referral, with Attorney-General George Brandis acknowledging Senator Gallagher had done nothing wrong.
The move puts pressures on a group of Labor MPs in the lower house who face similar eligibility questions.
Senator Gallagher said legal advice to Labor said she was eligible to sit as a member of Parliament.
"I have previously stated that I do not think there is a basis to refer my eligibility to the High Court for determination. I stand by that position.
"It is, however, clear to me that the government have decided I should be referred, despite having full access to all of my legal advice and expert reports for the past two days.
"A standard, I note, the government doesn't meet with their members of parliament on the disclosure register."
Senator Gallagher said she had taken the step amid attacks on her legitimacy.
"I have provided more information than anyone on my citizenship status, and anyone who reads the full disclosures can see for themselves the difference in standard and content that exists across this side of the chamber.
"While I do not agree with the need for this referral, I do not resent it.
"The success and standing of the Australian Senate is bigger than all of us, and should be focused solely on what we are able to deliver together for the Australian people.
"It is bigger and much more important than the circumstances of individual senators, and acting in a way which protects the reputation, the legitimacy, the confidence of our parliamentary institutions, should always, in my humble opinion, be paramount."
Labor's Senate leader Penny Wong said some MPs had their renunciations dealt with in a few days by UK authorities, but Senator Gallagher's case had taken four months.
"It is not within Senator Gallagher's power to determine the time it takes a foreign government to process her renunciation, and Labor is therefore confident that Senator Gallagher is constitutionally entitled.
"As Professor George Williams has stated this morning in the Sydney Morning Herald: 'It is difficult to see the court disqualifying a person because the bureaucracy of another country has taken months to process an application form. There is a low risk of these members offending section 44.'
"Senator Gallagher's eligibility is beyond question, so too is her integrity. Everybody in this place knows it."
Labor and the Greens used debate about the referral to put pressure on Prime Minister Malcolm Turnbull to refer a group of Liberal MPs who have questions on their eligibility.
If Senator Gallagher is found to be ineligible, she could be replaced by ticket mate David Smith, currently spokesman for Professionals Australia.
Mr Smith wouldn't comment on the saga this week but said he was eligible to serve in Parliament.