(Fixes typo in headline)

By Tom Westbrook

SYDNEY, Oct 30 (Reuters) - Australia's main Labor opposition party said on Monday it was considering a legal challenge to more than 100 decisions made by Prime Minister Malcolm Turnbull's conservative government, after two of his cabinet ministers were expelled from parliament.

Among the parliamentary votes in question was the government's defeat of a proposed wide-ranging inquiry into Australia's scandal-hit banking sector.

A powerful inquiry into Australia's banks, which are under fire after scams involving money-laundering, misleading financial advice, insurance fraud and interest-rate rigging, was approved by the upper house Senate this year, but fell one vote short of passing the lower house.

The government lost its one-seat lower house majority on Friday when a court ruled Deputy Prime Minister Barnaby Joyce was ineligible to sit in parliament as he had held dual citizenship when elected, in contradiction to the constitution. Former cabinet colleague Fiona Nash, along with three other politicians, were also expelled.

The ruling has cast doubt over the validity of parliamentary votes Joyce and Nash have cast, said Tanya Plibersek, acting leader of the Australian Labor Party

"We're going to look at all of our options," Plibersek said on Monday in an interview with Australian Broadcasting Corporation radio.

"We are very concerned about the fact that Barnaby Joyce has been voting at a time when he shouldn't have been in the Federal Parliament and we narrowly lost votes because of that."

Joyce has said many of those decisions were made by cabinet collectively rather than himself personally and so ought to remain valid.

Plibersek said legal challenges could be mounted by anyone aggrieved by the government's decisions.

She flagged challenging a narrowly-won vote that cut Sunday pay rates for some workers, as well as decisions taken by Joyce and Nash in their capacity as ministers. That includes a decision to relocate a government department headquarters to Joyce's electorate and decisions on water-rights allocation and the rollout of a national fast-internet scheme.

Constitutional law academic George Williams said the issue was not so clear cut. "It's certainly uncharted waters," said Williams, Dean of Law at the University of New South Wales.

The court would "look very carefully" in particular at decisions taken since August, when Joyce and Nash realised they may be dual nationals. "It's not clear what the result would be," he said.

The Australian constitution bars politicians with dual citizenship from being elected to the national parliament. Both Joyce and Nash said they were not aware they held dual citizenship and have since renounced their New Zealand and British citizenships, respectively.