The fact a third Greens Senate aspirant, Andrew Bartlett, is now in doubt due to the possibility he was "holding an office of profit under the Crown" at the time of his nomination indicates the party has a lot to learn when it comes to vetting candidates.

By apparently relying solely on the word of the individuals concerned, who it now transpires failed to exercise their own due diligence, the party of the moral high ground has made itself a laughing stock.

Prospective candidates for both houses of parliament receive information kits spelling out the boxes that need to be ticked to qualify.

Compliance with Section 44, which excludes citizens of a "foreign power", persons convicted of treason or for a crime punishable by 12 months jail, undischarged bankrupts and persons holding an office of profit under the Crown or working as Government contractors, is a big part of this.

One effect of the resignations has been to draw attention to the changes in Australia's demographics since Federation.

In 1901 Australian citizenship did not exist; the key criteria to which Section 44 applied was "British subject". As only 3.3 per cent of residents had been born outside of the Empire, native-born British subjects made up 96.7 per cent of the population.

Australians did not cease to be British subjects until after 1984.

By the time of the 2001 census 21.9 per cent of all Australians had been born overseas. We had become a diverse and multicultural society in which dual nationalities were a commonplace.

Something nobody would have thought could ever become a problem when the Constitution was drafted had emerged as a trap for young players.

That said, it is only fair to note that up until now only one other person, Heather Hill, a One Nation Senate candidate in 1998 from Queensland, has ever fallen foul of the "foreign power" provision.

Even though the section is outmoded and largely irrelevant given the oaths of office MPs and Senators are required to take, the vast majority of the thousands of people who have successfully stood for elected Federal office have managed to comply.

The difference between the Hill case and that of Ludlam and Waters is that neither of the latter were aware they held dual nationality.

Because Ms Hill never took her seat in the Senate the issue of repayment of salaries and allowances never arose.

It would be absurd, given the circumstances surrounding the current cases, to suggest the two Greens Senators, who served in good faith and with the full support of their constituents, should be bankrupted by such a demand.

But it is fair to say the Greens have some work to do to ensure such mistakes are not repeated in the future.