The forms making LNP senator Matt Canavan an Italian citizen are unsigned, according to Deputy Prime Minister Barnaby Joyce.

Senator Canavan has resigned from Cabinet but he is determined to fight to stay in Parliament and will ask the High Court to decide if he is eligible to keep his seat.

He revealed yesterday the Italian Government considered him to be an Italian citizen — which would make him ineligible to be in the Australian Parliament.

But Senator Canavan is arguing he should be able to stay.

"While I knew that my mother had become an Italian citizen, I had no knowledge that I myself had become an Italian citizen, nor had I requested to become an Italian citizen," he said.

Sorry, this video has expired Canavan family had citizenship discussion in 2005: Joyce

Mr Joyce will take on the ministerial roles Senator Canavan has given up.

He said this morning there was a discussion in the Canavan family in 2005, but, "from what has been conveyed to me, he had no interest in becoming an Italian citizen".

Labor leader Bill Shorten questioned how Senator Canavan came to be an Italian citizen.

"I think it would be useful if the Italian Consulate or the Italian Government, either directly or through Senator Canavan, could release all the documents," Mr Shorten told ABC Radio Melbourne.

"As I said, it's still got to go to the court, but, how does an adult become a citizen of another country without signing a form?"

During his media conference this morning, Mr Joyce staunchly defended Mr Canavan's character, describing him as an exemplary person.

Senator Canavan was previously Mr Joyce's chief of staff and Mr Joyce is godfather to the youngest of the Senator's five children.

"I'm very close to him. I've known him for years," Mr Joyce said.

He argued Senator Canavan might not be a valid Italian citizen, because it had been done without his knowledge or consent.

But the Deputy Prime Minister stumbled over his own situation when discussing the matter at a media conference this morning.

"My grandmother is English. I'm working on the presumption I've not English. I've never been to England," Mr Joyce said.

A short time later he tweeted, clarifying he had been to England but was not a citizen.

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Two Greens have quit parliament after discovering they were dual citizens.

Greens leader Richard Di Natale said it was now up to the High Court to determine if Senator Canavan will also be forced out of the Senate.

"If you're a real Italian, you never blame your mum for anything … and that might be his only defence in this case, I reckon," Senator Di Natale said.