That's according to lawyers for Peter Alley, the former Labor candidate who is challenging Assistant Health Minister David Gillespie's right to remain an MP.

Mr Alley believes Dr Gillespie may have an indirect financial stake in the Commonwealth, which is grounds for disqualification under section 44 of the constitution – the same section that has so far claimed nine MPs because of their dual citizenship status.

The Nationals MP for the seat of Lyne owns a small suburban shopping complex in Port Macquarie and one of the shops is an outlet of Australia Post, a government-owned business.

Dr Gillespie is seeking to quash the challenge by claiming the court is unable to rule on his eligibility because it was brought under the Common Informers Act. He and the government are arguing for the "primacy of Parliament" to make such referrals, as it has on citizenship.

But Mr Alley - represented by Bret Walker SC - says the court "can and must" proceed with the case, arguing the government-controlled House of Representatives could simply refuse to refer Dr Gillespie under the usual referral process.

"This court has the means of resolving the facts and their consequences relating to the defendant. Moreover, this court has both power and duty to do so," Mr Alley's submission to the court says.

"lt is difficult to conceive of a worthier public pursuit than an action which holds as its objective exposing a person as incapable of being a representative in the national parliament."

Mr Alley's submission points to former Attorney-General Kep Enderby's speech to Parliament when he introduced the common informer laws in 1975.

The laws were designed to prevent people profiting by bringing such cases but Mr Enderby was adamant that they would preserve the "basic right of all Australians" to bring a challenge.

He warned in particular of situations where a government could use its numbers in the lower house to prevent the referral of one of its own members. In that scenario, from the "point of view of the public there would appear to be something not very different from a conspiracy".

"The public would say 'these silly politicians are ganging up to protect one of their members'. They might use stronger words. The government took the view that there must be preserved an independent right to challenge a person's right to sit in this House," Mr Enderby said.

If Dr Gillespie is disqualified, it would spark a byelection in his NSW seat of Lyne.

The Nationals MP could run again if he disposed of the interest in the post office. The Nationals would be well-placed to retain the seat, although it was previously held by independent Rob Oakeshott.

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".

Sydney Morning Herald