Tensions over the by-election boiled over on Thursday with NSW Nationals leader John Barilaro calling on former Liberal prime minister Malcolm Turnbull to quit the party over his "treacherous" support for independent candidate Kirsty O'Connell.

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

"If the Liberal Party don't want to sack him, well be honourable and resign," he told reporters.

Mr Turnbull responded on Twitter saying: "What a joke! Barilaro called on me to resign when I was the Liberal Prime Minister of Australia!"

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

A win in Saturday's by-election is crucial for the Berejiklian government, which is already in minority after losing four MPs to the cross bench.

The Nationals hold the seat with a 2.6 per cent margin but last election polled just 34 per cent of first preference votes to Labor's 28.6 per cent, with the Shooter Fishers and Farmers attracting 22 per cent.

Polling last week found there was a razor thin margin between the leading candidates in the coal mining and agricultural community.

Voters will be confronted by a choice of 10 candidates and have the option of nominating one or all candidates in descending order.

Mr Barilaro is urging Nationals' voters to select just one candidate "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".

The Nationals failed to get Mrs O'Connell's preferences and Mr Barilaro says he 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," Mr Barilaro told Sydney Radio 2GB.

Mrs O'Connell said she had approached NSW Nationals state director Joe Lundy and "asked him for sensible policy shifts that any true Nat should have been happy to support but they refused to show any common sense".

"I asked them to at least pause all new mining, gas and exploration while we make a sensible plan for our future. The Nats refused to help," she said in a statement.

"The Nats told me they had no room to move on policy at all - yet last night they announced that they will form an expert panel to advise on transition. If they were so interested in my support why wouldn't they have a sensible conversation weeks ago?"

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 Mrs O'Connell is 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".

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.

Australian Associated Press