The Greens leader believes two of his former senators shouldn't have to pay back their parliamentary salaries despite being ineligible to have served.
Larrisa Waters on Tuesday became the second Greens senator in a week to resign after discovering she was a dual citizen - something not allowed under the constitution.
Richard Di Natale who has ordered a root and branch review of the party's processes and insists the party vets all of its candidates and both indicated they weren't dual citzens.
"We should have triple checked and we didn't," he told ABC radio on Wednesday.
The rest of the Greens party, which includes the Singaporean-born Peter Whish-Wilson and Nick McKim who was born in the UK - have checked their citizenship status.
Senator Di Natale doesn't believe Ms Waters and Mr Ludlam should have to pay back their salaries or be treated differently to former senator Bob Day, whose debt was waived.
"They've already paid a pretty high price," he said.
The party leader expects the next candidates in line to fill the gaps - Jordon Steele-John in WA and former Democrats leader Andrew Bartlett in Queensland - will make a great contribution.
He described Mr Steele-John, a youth and disability advocate, as a "force of nature".
But he hopes Ms Waters and Mr Ludlam return to parliament down the track, and is confident if they don't they will make a contribution to public life elsewhere.
One of his predecessor's Bob Brown also hopes they run again sometime.
"I think they're very good parliamentarians and they're very popular and they've added something to the Greens," he told ABC radio.
The Greens, however, had a deep pool of talent, he said.
Prime Minister Malcolm Turnbull said the cases of Ms Waters and Mr Ludlam showed incredible sloppiness.
But he confirmed they would be paired while their vacancies were filled.
"The seats won't be vacant for long," he told the Nine Network.
