The decision could have serious ramifications on four other MPs with similar citizenship issue, who were relying on the case.
Gallagher was aware of her British passport as her father was born in England, so the court ruled that she did not take "all reasonable steps" to relinquish her British links, Xinhua news agency reported.
Under Section 44 of the Australian Constitution, members of parliament cannot continue to have citizenship in another country, and must prove they took "all reasonable steps" to renounce the other citizenship before being elected.
The high court's precedent could also prompt the resignation of -- Susan Lamb, Justine Keay, Josh Wilson and Rebekha Sharkie --, who were dual citizens when they were elected in 2016.
Lamb, Keay and Wilson are Labour Party MPs, while Sharkie is a conservative MP with the Nick Xenophon Team.
Gallagher's seat in the Senate will be taken by David Smith, the second Labour candidate for the Australian Capital Territory.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
