Senators are expected to ask the High Court on Monday if the election of two of their colleagues breached the constitution.
The government has taken legal advice casting a cloud over the election of former Family First senator Bob Day and One Nation's Rod Culleton.
Mr Day resigned to deal with his construction company collapse last week, hours before the government publicly revealed it would push to challenge his election on the grounds he had an indirect pecuniary interest arising from the lease arrangements for his Adelaide electorate office.
The government argues Senator Culleton's election is invalid because he was convicted of larceny - since annulled - at the time of the federal election.
The constitution bars anyone convicted of a crime attracting at least 12 months' prison time from being elected.
The Senate is expected to deal with the matters early on Monday morning, after new Labor senator Kimberley Kitching is sworn in to replace the retired Stephen Conroy.
Labor has said it would support the government in referring the constitutional questions to the High Court.
