ACT Labor senator Katy Gallagher is expected to face a High Court ruling on her eligibility to sit in parliament on Wednesday, six months after it emerged she was a dual citizen at the 2016 election.

A decision against the former ACT chief minister may not lead to a long stint outside parliament, as Labor readies to preselect its candidates for both the Senate and lower house later this year.

The court's decision could see her leave the Senate and may cast doubt on the parliamentary futures of other Labor MPs facing questions over their eligibility.

Labor has waited to learn Senator Gallagher's future since a court hearing in March and the High Court has been forming its decision while the possible new boundaries of a third ACT electorate were released.

Professionals Australia's ACT director David Smith would likely take Senator Gallagher's seat if the High Court finds her ineligible.

There is speculation she could run for one of the lower house seats if ejected from the Senate, but she may have to wait only until August for Labor preselections for lower and upper house candidates, a month after new seat boundaries are finalised.