A group of Nationals members have quit the party amid an investigation into their alleged connection to alt-right, neo-Nazi and white supremacist groups.

A member of the Young Nationals executive sent a lengthy letter to the party on Wednesday, accusing it of trial by media while also accusing the Coalition of “opening Australia to mass third-world immigration”.

The man, who was implicated in revelations about an alt-right infiltration of the party earlier this month, said he and about 15 others currently under investigation would tender their resignations.

“I have not been given the right of reply or a chance to explain,” the man said. “This is very much a Kafkaesque situation right out of The Trial.”

AAP understands the former member attached another document to his letter containing a list of the 15 members who also tendered their resignations.

The man denied he and his colleagues had been making a known white supremacist hand gesture, instead suggesting it could be interpreted “as declaring a point or saying ‘bon appetite’”.

He also launched a scathing attack on the Coalition for its current immigration intake, saying “opening Australia to mass third-world immigration is not ‘moderate’. It’s extremist.”

The ABC, which conducted the original investigation into the issue, reported the letter also said “young white Australians” were rebelling against “hopeless Coalition leadership” and feared becoming “a minority in their own country”.

It was reported on Wednesday the supposed ringleader of the branch-stacking operation would not be investigated by the organisation’s ethics committee.

It is understood the Nationals’ ethics committee would meet on Thursday to further investigate the remaining members of the party caught up in the saga.

The Nationals frontbencher Darren Chester has warned his party must be vigilant with membership applications.

“We’ve got to be careful in the sense of our vetting and checking, making sure you don’t get those type of characters trying to infiltrate your party,” Chester told ABC radio on Thursday.

He said a process was under way in NSW to ensure it would not happen again.

“I’m disappointed it’s got to this,” Chester said. “There’s some people obviously who have tried to use the party for their own agenda.”

The former member attached a list of the 15 members who also tendered their resignations.