A Liberal-National Party senator has denied benefiting from federal government road contracts.
Under section 44 of the constitution, anyone who holds "any direct or indirect pecuniary interest in any agreement with the public service of the commonwealth" is disqualified from running for parliament.
Queensland senator Barry O'Sullivan has faced questions from Labor about his ties to a company called Newlands Civil Construction.
The register of senators' interests shows Senator O'Sullivan owns a portion of a company called Jilbridge Pty Ltd.
Jilbridge has a share of O'Sullivan and Sons Pty Ltd, which - until the senator made changes to the register on Monday - was a shareholder in Newlands Civil Construction, a company managed by the senator's son.
In August it emerged Newlands had a four-project contract with Nexus Infrastructure - one of the partners on the Toowoomba Second Range Crossing, a $1.6 billion road bypass.
The federal government is contributing $1.14 billion, or 80 per cent of the project cost, and the Queensland government's share is 20 per cent through a public-private partnership contract.
It is understood the federal funding is combined with state money under the Queensland Transport and Roads Investment Program, and the Queensland government makes payments to Nexus under the PPP contract.
Newlands also has a number of other road contracts.
Senator O'Sullivan told parliament on Thursday both Finance Minister Mathias Cormann and Transport Minister Darren Chester had searched departmental records and found no evidence of Newlands having contracts with the commonwealth.
But in any case, he said he had no financial interest in Newlands.
He challenged Labor to find any evidence to the contrary.
"If they have contracts, a piece of paper, an affidavit, something on the back of a napkin, that says I have a direct or indirect interest in anything to do with contracts with the commonwealth of Australia I call on them to table those documents or supporting evidence to this place," Senator O'Sullivan said.
"They do not exist."
Senator O'Sullivan said the saga had had a "serious effect" on his family and the reputation of Newlands, which employed more than 120 people.
"I do not intend to respond any more to any allegation made by the opposition or anyone else in this place."