Mr Barilaro addressed a buoyant crowd at Singleton Diggers on Saturday night after his "second pick" to contest the by-election, Clarence Town engineer Dave Layzell, was forecast to retain the seat with a slightly improved majority.

The Nationals had won 31.2 per cent of the primary vote with 44 per cent of the ballots counted.

Labor's vote had collapsed to less than 21 per cent, seven percentage points below their performance in the 2019 election.

Almost half the electorate, or 23,000 people, voted before election day, and Labor's Jeff Drayton is expected to gain ground on the Nationals when most of those votes are added to the tally in the next two days.

But the pre-poll count is not expected to change the result.

The Labor campaign said winning would be "tough for us from here" but was not conceding victory.

Mr Barilaro sarcastically thanked former Liberal prime minister Malcolm Turnbull, who endorsed independent Kirsty O'Connell and her call for a moratorium on new Hunter coalmines, for helping the Nationals' campaign.

"Thank you, Malcolm," he yelled to the crowd.

"We're not going to claim 100 per cent tonight.

"As you know, there's still Muswellbrook pre-poll to come ... but there's no doubt we've got a substantial lead.

"Something would have to go drastically wrong from here, so we are confident we've got the victory."

Mr Layzell thanked "our rock star, Gladys", in reference to Premier Gladys Berejiklian.

If the result holds, it will be a rare by-election victory for a sitting government and an even rarer swing in its favour.

The Nationals had a huge contingent of 400 campaign workers on the ground at polling booths around the electorate on election day.

"We out-campaigned Labor," Mr Barilaro said.

Mr Drayton said he had "never seen anything like it" after encountering "about 40" Nationals campaign workers at a Singleton polling booth.

The Nationals had Deputy Prime Minister Michael McCormack in Quirindi and a host of other Coalition luminaries, including Ms Berejiklian, Treasurer Dominic Perrottet and New England federal MP Barnaby Joyce, campaigning in the electorate on polling day.

Labor had NSW leader Jodi McKay, Hunter federal MP Joel Fitzgibbon and most, if not all, its state caucus on the ground.

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Based on votes counted, One Nation's Dale McNamara (13.8 per cent) outperformed the Shooters' Sue Gilroy (12.9 per cent).

The Shooters vote was down more than eight percentage points on 2019.

Ms O'Connell won 8.4 per cent of the primary vote, former Dungog mayor Tracy Norman 3.8 per cent and the Greens' Sue Abbott 3.2 per cent.

Primary vote so far

Labor's campaign focused on shoring up its traditional mineworker base in Singleton and Muswellbrook, where the party won booths in the 2019 general election.

Mr Souris told the ABC during the day that he believed the Nationals' "natural" margin in Upper Hunter was larger than what recently resigned MP Michael Johnsen had achieved.

Mr Johnsen quit Parliament in March over allegations he sent lewd messages to a sex worker during question time.

He is also facing a police investigation into allegations he sexually assaulted the sex worker in the Blue Mountains in 2019. He strongly denies the allegations.

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