* Government defeats push for court to scrutinise its MPs citizenship
* High Court's review of opposition senator's status seen as test case
* All eyes on Dec. 19 by-election in Sydney (Adds second MP referred to High Court, defeat of proposal to refer more MPs)
By Tom Westbrook
SYDNEY, Dec 6 (Reuters) - Australia faces a series of by-elections that could topple the government, which trails in opinion polls and has lost its slender majority, in a bizarre citizenship crisis that has engulfed both sides of parliament.
Senator Katy Gallagher and lower-house member David Feeney, both from the opposition Labor party, were referred to the High Court on Wednesday to determine whether they hold British, as well as Australian, citizenship.
Neither is a member of the government, but the outcome of Gallagher's case in particular, which rests on whether she made "reasonable steps" to renounce her British citizenship, will set a precedent that could later unseat government members.
Dual citizens are ineligible for elected office under Australia's 116-year-old constitution.
In a nation in which half the population were either born overseas or have parents who were, the rule has disqualified nine lawmakers, and left Prime Minister Malcolm Turnbull's Liberal-National coalition clinging to a minority government. Prime Minister Barnaby Joyce briefly lost his seat when it was found he also held New Zealand citizenship. But he won it back in a by-election last weekend. by-election in former tennis star John Alexander's theoretically safe Sydney seat on Dec. 19 will determine whether the government regains its one-seat majority.
However, a victory may not be lasting, since the citizenship status of another four lower-house government lawmakers was called into question after a deadline for politicians to disclose the birthplace of their parents and grandparents passed on Tuesday.
"There are many inadequate disclosures that ask more questions than provide answers," Labor leader Bill Shorten told reporters in Canberra.
'TEST CASE'
The government, behind in opinion polls and keen to avoid any more by-elections, voted down a Labor proposal to refer those lawmakers' cases to the High Court and said it would not revisit the matter until after the Gallagher case was heard.
Gallagher filed paperwork, and paid processing fees, to renounce her British citizenship more than two months before being elected in July 2016.
But she did not get confirmation from the British Home Office that her renunciation had been processed until after she was voted in, her disclosure documents show.
Several other lawmakers are in a similar bind.
"It will be a test case," Constitutional law expert George Williams, dean of law at the University of New South Wales, told Reuters by telephone.
"But it leaves open the possibility that now this will go on for quite some time. There's large question marks over quite a number of people."
Turnbull's government would have to rely on the support of a handful of independent MPs to retain power if Alexander his Dec. 19 by-election, or if the High Court ousts another coalition lawmaker from the lower house.
"It's uncertain territory, we still don't know for sure who is eligible and who's not," University of Queensland politics lecturer Chris Salisbury told Reuters.