The Greens are reeling after a second senator within a week, Larissa Waters, was forced to resign from parliament for being a citizen of two countries.

The abrupt departure was announced yesterday after learning that because she was born in Canada, albeit to Australian parents, and moved Down Under at 11 months of age, she was a dual citizen.

Ms Waters’ announcement came just days after colleague Scott Ludlam resigned when he discovered he held both Australian and New Zealand citizenship.

Under Section 44 of the constitution, dual citizens are not permitted to sit in parliament. The rules could not be more straightforward.

Yet, for a combined 16 years, these politicians have been paid very handsomely – millions of dollars, in fact – for holding positions within the parliament that they were never entitled to hold.

But even more important than the money is the influence these two senators have been allowed to exert over legislation, policy and debate within this country.

The mistakes made by Ms Waters and Mr Ludlam might have been innocent and inadvertent, however, the consequences of their incompetence are significant.

The Australian Greens are not a new, small or poorly resourced party; they have been a fixture on the national political scene for decades.

There is no excuse for them failing to vet their candidates – particularly Senate candidates – to ensure they are eligible to stand for election.

The Greens are unquestionably at the lowest ebb of their history.

Even before their co-deputy leaders were forced into deeply embarrassing resignations, trouble was brewing under the leadership of Richard Di Natale.

A disappointing performance at the 2016 election was followed by internal ructions that saw senator Lee Rhiannon suspended from the party room last month.

The Greens have played an important role in bringing pressure to bear on the major parties on numerous environmental and social issues over many years.

They still have the capacity to be a force for good well into the future, but they must get their act together now or risk becoming obsolete.

- Ross Tyson, deputy editor