The High Court is set to decide whether a senator who quit parliament over the dual citizenship fiasco is eligible to reclaim her job.
Former Nick Xenophon Team senator Skye Kakoschke-Moore stepped down in November over her British links.
The South Australian was due to be replaced by the next candidate on the NXT senate ticket at the 2016 election, Tim Storer.
But Mr Storer has quit the party and Ms Kakoschke-Moore's legal team says this renders him ineligible to take the seat.
Her lawyers also argue she should be allowed to take back her seat because she has since severed her British ties.
The case will be heard by the full bench of the High Court in Canberra on Tuesday.