Fairfax Media revealed in February that Dr Gillespie may have an indirect financial interest in the Commonwealth - grounds for disqualification under section 44(v) of the constitution.

Dr Gillespie, the assistant health minister, owns a small suburban shopping complex at Lighthouse Beach in Port Macquarie. One of the shops within the complex is an outlet of Australia Post – a government-owned corporation.

Labor first wrote to Attorney-General George Brandis in February asking whether he had sought legal advice about Dr Gillespie following Fairfax Media's report, noting the Turnbull government's lower house majority was on the line.

Shadow attorney-general Mark Dreyfus has now escalated the "very grave matter" to Prime Minister Malcolm Turnbull, noting similarities to the case of Mr Day, whom the High Court ruled ineligible to be elected on Wednesday.

The South Australian property developer, who had already from resigned his Senate position last year, was found to have an indirect pecuniary interest in the Commonwealth owing to the structure of his Adelaide electorate office.

Mr Dreyfus said there was a "clear correlation" between the two cases, and called on Senator Brandis to reveal any legal advice he had obtained from Solicitor-General Stephen Donaghue about the "highly concerning situation".

"Now that the High Court has clarified the law, it is urgent that the A-G obtain advice on Mr Gillespie's eligibility," Mr Dreyfus told Fairfax Media. "This is a finely-balanced Parliament in which every vote counts."

Constitutional expert George Williams at the University of New South Wales agreed the matter should be investigated.

"Very clearly, now is the time to get advice on this arrangement," he said. "It is appropriate that the Solicitor-General's advice is sought in light of the Day decision."

Anne Twomey at the University of Sydney said Dr Gillespie's case would depend on the facts, but if the commercial transaction were at "arms-length" and involved no conceivable influence over his public duties, then "it should be OK".

Section 44(v) is an anti-corruption measure, designed to stop people sitting in Parliament and at the same time making money through contracts with the Commonwealth. But it is a murky provision that has rarely been tested in court.

It would be highly unusual for the government to release legal advice sought on such a matter, or confirm if a legal investigation were under way.

A spokesman for Senator Brandis declined to comment on Thursday night. Dr Gillespie did not respond to requests for comment.