Labor senator Katy Gallagher's appointment to the federal Upper House will be investigated by an ACT Legislative Assembly committee, amid the growing citizenship saga.

Senator Gallagher was appointed by the assembly to fill a casual Senate vacancy in 2015.

At the time, she signed a statutory declaration, presented to the assembly, which said she was not a citizen of a foreign country.

But questions have been raised about whether Senator Gallagher was at the time a dual citizen by descent, because Senator Gallagher's mother was born in Ecuador to British parents who were working in the country temporarily.

The former ACT chief minister told the Senate in September she renounced any entitlement of British citizenship before the 2016 federal election — a year after she joined federal parliament.

The Canberra Liberals on Tuesday called for an investigation by a new committee with the power to compel witnesses to appear into the statutory declaration presented to the assembly.

That has been rejected and instead the existing administration and procedure committee will investigate both of the chamber's previous Senate appointments — Senator Gallagher's in 2015 and former Liberals' senator Gary Humphries in 2003.

It will look at whether "previous appointments made by the assembly might be considered in hindsight to be unsound".

The committee will also report on whether the assembly should adopt new practices in appointing senators.

This is not a witch hunt: Liberals

The motion, which passed with the support of Labor, the Liberals and the Greens, was originally moved by Liberal MLA Vicki Dunne.

"We all have been aware of the issues relating to citizenship ... and the implications that has had for many [federal] members," Ms Dunne told the assembly.

"This is not a witch hunt ... this is an issue about the soundness and the safeness of decisions made in this place and to ensure we have a robust mechanism for dealing with casual vacancies."

Greens leader Shane Rattenbury said processes needed a rethink in the wake of the citizenship saga.

"People's common understanding of eligibility for Federal Parliament has been changed by the circumstances of the past six months," he said.

"I think it is the right pathway for the administration and procedures committee to have a look at this and see whether the assembly needs to reconsider the role that it plays."

While Labor supported the motion, the party was quick to point out that the motion was targeted at making sure future processes were robust — not raking over previous appointments.

"This is not seeking to put itself in the place of the High Court to determine the eligibility or otherwise of Senator Gallagher," Chief Minister Andrew Barr said.

"This is about the future, not about the past."

The committee is due to report in March next year.