Assistant Health Minister David Gillespie's political future is on the line - and so is the government's one-seat majority - after Labor launched a constitutional challenge against his eligibility last week.

The opposition believes he may have an indirect financial interest in the Commonwealth - grounds for disqualification from office under section 44 of the constitution - because he leases a commercial space to an Australia Post licensee at Lighthouse Beach, near the NSW coastal city of Port Macquarie.

If the High Court finds he was invalidly elected it will spark a by-election in the National MP's seat of Lyne and cause a massive political headache for Prime Minister Malcolm Turnbull.

If Dr Gillespie's election is ruled invalid, it is believed he may be able to stand again in at a by-election and re-win the seat - but not if he retains the offending interest.

Now the business has been listed for sale with a price of $450,000. But Dr Gillespie's office says his tenant has made the private decision to sell up and he did not request it or have any conversations with them about the move.

In the for sale ad, Lighthouse Beach is described as a "most pleasant location to visit and live" and the business "perfect for a husband and wife team".

It brings in a net income of approximately $175,000 a year.

"Trading in its current location for many years, fitted out to Australia Post requirements and specification. The shop is large with room to introduce other products," the listing reads. "This shop has a strong and loyal local client base. The current owners have been operating the business for over 27 years."

Nationals Leader Barnaby Joyce said he had spoken with his MP about the court challenge but did not believe Dr Gillespie would have an issue given the financial interest was so indirect.

Dr Gillespie and his wife, through their company Goldenboot, lease the shop space in question to a woman who is an Australia Post licensee - meaning he has no direct financial link to the government-owned postal service.

Mr Joyce also pointed out the Dr Gillespie - who owns 18 properties - did not enter politics for the money.

"This guy is a gastroenterologist. He would have taken a serious pay cut to go into politics. He's doing it for the right reasons," he told the ABC.

"He's a strong member of the community. He's doing a good job in health and that's precisely what we want."

Section 44(v) of the constitution says any person who "has any direct or indirect pecuniary interest in any agreement with the Public Service of the Commonwealth otherwise than as a member and in common with the other members of an incorporated company consisting of more than 25 persons shall be incapable of being chosen or of sitting as a senator or a member of the House of Representatives".

The section is an anti-corruption measure, designed to stop people sitting in Parliament and at the same time making money through contracts with the Commonwealth.

Follow us on Facebook