Government backbencher Barry O'Sullivan may have fallen foul of the constitution because of family business dealings.

The Queensland senator's family's construction company Newlands Civil Construction is subcontracted to work on a federally funded project - the Toowoomba second range crossing.

The contract, worth $2.5 million, 80 per cent of which comes from Canberra, covers demolition work, sewerage diversion, water mains and site access gates, according to the company's website.

There are question marks over whether Senator O'Sullivan is in breach of section 44 of the constitution, which states a member or senator can be disqualified where there is "any direct or indirect pecuniary interest in any agreement with the public service of the Commonwealth".

AAP understands Newlands is subcontracted by another company called Nexus which has an agreement with the Queensland state government.

Senator O'Sullivan denies he "either directly or indirectly, holds a pecuniary interest in any agreement with the public service of the Commonwealth."

He insists his interests are published publicly, in their entirety, and methodical effort is taken to ensure these details are updated thoroughly.

"Importantly, a review by Newlands Civil Construction has proven that it does not hold any agreement with any company or entity that has an agreement with the public service of the Commonwealth," he said in a statement.

He pointed the media to his register of interests which was last updated in May.

On the second page under the section shareholdings in public and private companies, Senator O'Sullivan lists his interest in Jilbridge Pty Ltd which is a subsidiary of O'Sullivan and Son Pty Ltd which is a subsidiary of Newlands Civil Construction Pty Ltd.

But he insists he does not currently have a direct interest in Newlands.

Senator O'Sullivan's son, who is also named Barry, is the managing director of Newlands.

"The O'Sullivan family remains the controlling shareholders of the group," his Linkedin profile states.

"Newlands has grown from a small family operation to an organisation that now employs in excess of 100 staff with additional management oversight of up to 60 independent subcontractors."

In April, the High Court found Family First senator Bob Day was ineligible to sit in parliament because he indirectly benefited from the government's lease arrangement on his Adelaide electorate office.