Labor senator Katy Gallagher's lawyers say she took "every step" to renounce her British citizenship within the proper timeframe and should remain eligible to sit in parliament.
Senator Gallagher will face the High Court on March 14 which will determine if she is disqualified from sitting in parliament because of her dual citizenship.
In a written submission released on Tuesday, her lawyers say she completed a declaration of renunciation of her British citizenship on April 20, 2016, and the UK Home Office deducted a fee for the registration on May 6 - 10 days before the issuing of writs for the federal election.
The lawyers cite experts saying the material provided on April 20 was "sufficient there and then to satisfy the requirements imposed by the law of Britain for cessation of (Senator Gallagher's) citizenship".
However, the UK Home Office asked Senator Gallagher on July 20, weeks after election day, to provide originals of her father's birth certificate, her parents' marriage certificate and her own birth certificate.
The lawyers said providing these extra documents was "an additional layer of proof" of the information already provided, and something not reasonably required under British law.
They said the test set out in the High Court case which cleared Matt Canavan of doubts over his eligibility had not been a "reasonable steps" test, but rather "where it can be demonstrated that the person has taken all steps that are reasonably required ... and are within his or her power".
Australian Associated Press