The problems of Scott Ludlam, Larissa Waters and now Matt Canavan are over-simplified by saying there is a constitutional ban on dual citizens serving as members of parliament or standing for election.

In fact, the constitution does not make reference to "dual citizenship" and the constitutional issues involved are not entirely clear.

All three senators have fallen foul of Section 44(i) of the constitution that sets out a rather archaic definition of foreign allegiance.

It has the potential to make it impossible for some Australian citizens to serve in the Australian Parliament or even contest election.

The difference in Senator Canavan's case is that he is entitled to citizenship by descent rather than by birth, a category of citizenship that the High Court has not previously ruled on.

The first part of Section 44(i) sets out a disqualification for any person who:

"Is under any acknowledgment of allegiance, obedience, or adherence to a foreign power, or is a subject or a citizen [of a foreign power]."

This is the provision that has caught out Scott Ludlam and Larissa Waters. Both were born in foreign countries and therefore had foreign citizenship by birth.

Both assert they were unaware of their dual citizenship, but by a previous court ruling both fall foul of Section 44(i).

High Court disqualified two naturalised Australian citizens

The previous case was Sykes v Cleary in 1992.

The case disqualified Independent MP for Wills Phil Cleary for holding an "office of profit under the Crown" by Section 44(iv) of the constitution.

This is another archaic part of Section 44, but I won't delve into its difficulties here.

The Sykes v Cleary judgement also examined the citizenship status of Cleary's election opponents.

The Liberal candidate was born in Switzerland, was a naturalised Australian citizen, but still had Swiss citizenship. The Labor candidate was Greek born and caught in the same trap: a naturalised Australian but still possessing Greek citizenship.

The majority of the High Court ruled that both were disqualified as they had not attempted to rid themselves of their foreign citizenship.

The Court acknowledged that in some cases Australian citizens cannot rid themselves of foreign citizenship.

Rather than let foreign law define whether an Australian citizen can run for Parliament, the High Court adopted a test in Australian law that a candidate must make "all reasonable steps" to rid themselves of other allegiances.

Which is where Mr Ludlam and Ms Waters ran into problems. Neither had taken steps to rid themselves of their citizenship by birth.

Both appear to be relatively clear extensions of the Sykes v Cleary ruling.

Senator Canavan is in a different category stemming from the second half of Section 44(i) which reads:

"Or is entitled to the rights or privileges of a subject or a citizen of a foreign power."

Senator Canavan's mother applied for Italian citizenship in 2006, and at the same time applied for her children, including Senator Canavan, who was 25 at the time.

Senator Canavan states he did not know of this application, has confirmed with the Italian authorities that he did not make the application, but has been informed that by that application he is a foreign resident Italian citizen.

But let's not go into the issue of the application having been made without his consent.

The matter here is that the grant has confirmed that Senator Canavan is "entitled" under the second part of Section 44(i).

Sorry, this video has expired Matt Canavan resigns from ministry over citizenship doubts (Photo by AAP: Lukas Coch)

Canavan's case will set benchmark for candidates

The High Court has not ruled on this sort of case, and the High Court's consideration will set a precedent.

Mr Ludlam and Ms Waters might now be encouraged to fight their own disqualification in conjunction with the Canavan case.

There are vast numbers of Australians who have an entitlement to foreign citizenship. If they apply to claim that foreign citizenship by providing relevant documentation, then they clearly fall foul of the test in Sykes v Cleary.

But what if you have an entitlement that you have never activated, or in some cases are not even aware of? Can a right you do not know of and have not applied for disqualify you under Section 44(i)?

Clearly there is some limit to how far Section 44(i) can extend.

If Vladimir Putin granted Malcolm Turnbull all rights as a Russian citizen, it would be ridiculous to argue it would disqualify Mr Turnbull from sitting in the Australian Parliament. The High Court mentioned such a hypothetical example in the Sykes v Cleary ruling.

Senator Canavan's case will set a benchmark on the rights of Australian citizens to run for Parliament.

Will the Court extend its Sykes v Cleary judgment to apply to citizenship by inheritance, or will it make a more sensible test concerning whether an Australian citizen had attempted to claim their foreign right?

Alternatively, the High Court could step back from its previous rulings. In both Sykes v Cleary and the later Sue v Hill case, the minority of justice were of the view that taking out Australian citizenship and rejecting all previous allegiances was enough.

And in between those two positions are many shades of grey that this High Court case might help illuminate.