NSW National Party leader John Barilaro has savaged former Liberal prime minister Malcolm Turnbull over his support for an independent candidate in the crucial Upper Hunter byelection.

“On the eve of this election … he continues to show this level of treachery,” he told 2GB radio on Thursday.

The NSW Coalition will be plunged into a minority government if it loses the highly anticipated election.

Mr Barilaro is urging Nationals’ voters to vote one on Saturday only “to make sure that your preferences aren’t being skewed to a candidate that you don’t have any idea of or any will to support”.

Mr Turnbull, who owns a farm in the Hunter Valley, is backing anti-coal farmer Kirsty O’Connell and has reportedly donated $3000 to her campaign – the maximum allowed for independent candidates under electoral commission rules.

Polling suggests the result in the previously blue-ribbon Nationals seat in the coal mining community will be decided on preferences with a razor-thin margin between the leading candidates.

Mr Barilaro said he had tried and failed to get Ms O’Connell to preference the Nationals and now fears “the ugly web of preferences” could see her supporters’ preferences go to Labor.

“She asked for a moratorium on coal …. she’s anti-mining and anti gas … and that was the end of that conversation,” he said.

Mr Barilaro has previously criticised Mr Turnbull for lecturing regional residents on the need for a mining moratorium.

“What they want is a response by a government that understands there is a transition needed in the decades ahead; we’re prepared to do something today to start laying the foundations, but we don’t scare people about their jobs.”

The YouGov poll last week found 25 per cent of voters support Nationals candidate Dave Layzell while Labor’s Jeff Drayton is sitting on 23 per cent.

Support for Ms O’Connell is just six per cent and she has declined to preference the major parties.

However, Labor is hoping to pick up her preferences with brochures saying: “Voting Kirsty O’Connell? Vote 2 Jeff Drayton”.

A furious Mr Barilaro accused Mr Turnbull of wanting “to see the Berejiklian/Barilaro Liberal National Coalition government go into minority”.

“He’s just showing his treacherous colours once again,” he said.

“This is his track record. He’s a destroyer not a builder.”

The by-election was sparked by Nationals MP Michael Johnsen’s resignation in March after being accused of raping a sex worker in 2019.

Mr Johnsen denies the allegations and has not been charged.

Last week a fourth government MP was forced to move to the crossbench, leaving the Berejiklian government in minority.

-with AAP