At least 15 members of the NSW Young Nationals have resigned following an investigation by the ABC's Background Briefing program into an alt-right push inside the party.

They include at least four men identified in the investigation as being involved in a plot by Australia's alt-right movement to join and influence major political parties.

Those men are: Clifford Jennings, Oscar Tuckfield, Michael Heaney and Thomas Brasher.

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The group resignations were sent in two letters emailed to the NSW executive of the National Party on Wednesday afternoon.

One letter, titled "resignation in protest", listed the names of the 15 Young Nationals and announced: "There will be a few more to follow".

The other letter, "An Open Defence, by Clifford Jennings" accused the National Party of being complicit in a "trial by media" and questioned the party's immigration policy.

Mr Jennings, the party's former coordinator for Sydney metro region, wrote at length on his alleged treatment by the party, with the three-page letter outlining a number of complaints.

The letter also delved into immigration policy, with Mr Clifford writing "young white Australians" were rebelling against the "hopeless Coalition leadership that has dragged down Australia since the 1970s".

Clifford Jennings (third from the right at the back) poses for a Young Nationals group photo. ( Facebook: Young Natioanls )

"They see a grim future for themselves and their children, of becoming a minority in their own country," Mr Clifford wrote.

"Opening Australia to mass third-world immigration is not 'moderate'. It is extremist.

"The fact that this policy has been followed by the Coalition parties for the last four decades does not make it moderate. It is treasonous."

The NSW National Party confirmed it had received the resignation letter. In a statement it said an internal investigation into the reported "far-right infiltration of the party" would continue.

"As previously advised, a statement will be released to media upon completion of that investigation," it said.

The ABC understands that statement will be released in a matter of days.

The Background Briefing report from earlier in October revealed members of the NSW Young Nationals had been attending a secret men's only fight club set up by some of the country's most prominent alt-right nationalists.

It also revealed the existence of a Facebook group, called The New Guard, where members of the Young Nationals who described themselves as fascists shared plans to shake up mainstream politics.

More NSW Young Nationals were found to be sharing alt-right jokes, references and conspiracy theories online.