Some 13 would-be parliamentarians are making their last pitches to the people of the Upper Hunter before polls open at 8am on Saturday.

While Labor candidate Jeff Drayton talked up his party's promises and sought to differentiate himself from the abundance of coal-loving candidates, Nationals Leader John Barilaro's message was simple.

"Don't change that jockey mid-race," Mr Barilaro said on Friday alongside the party's candidate, construction manager David Layzell.

"In 18 months (at the 2023 election), if we haven't done a good job by then, of course, send us a message.

"But now's not the time."

Results are expected to be tight despite the Nationals and its predecessor party holding the electorate for nearly a century.

The poll was sparked by Nationals MP Michael Johnsen's resignation in March over sexual assault allegations and a sexting scandal.

Mr Johnsen denies the allegations and has not been charged.

The seat was whittled down to a 2.6 per cent margin in 2019, when Mr Johnsen scooped up 34 per cent of first preferences.

Preferences are again likely to decide the contest for the seat, which takes in Singleton, Gloucester and Muswellbrook.

The Nationals' campaign has hinged on the need for stable government but the other players have focused on the future of the mining industry, which employs 15 per cent of the electorate's residents.

Candidates have been falling over themselves to declare their support for coal, but Labor's Mr Drayton says he's the only one backing coal workers too.

"It's alright to support mine companies but ... mining is no good to us here in the Hunter Valley unless we support mine workers," the former coal miner and CMFEU leader told reporters on Friday.

Labor has pledged to return a greater share of mining royalties to local communities, and to introduce bills fighting workforce casualisation and toughening industrial manslaughter penalties.

Mr Drayton says he is confident the legislation, which would need crossbench support, would pass parliament if he was elected.

Shooters, Fishers and Farmers Party candidate Sue Gilroy and One Nation pick Dale McNamara have also touted their coal mining credentials, while high-profile independent and fifth-generation farmer Kirsty O'Connell sits on the other end of the spectrum.

Ms O'Connell has drawn the support of former prime minister Malcolm Turnbull, who owns property in the Hunter, over her support for a moratorium on coal mines.

Greens candidate Sue Abbott is also running on a promise to transition the area away from reliance on coal.

But recent polling suggests the race will come down to Mr Layzell and Mr Drayton, with a wafer-thin margin between them.

The YouGov poll of 400 people conducted in the Upper Hunter last week found 25 per cent support for the Nationals candidate and 23 per cent for Labor.

SFF candidate Ms Gilroy has 16 per cent support, One Nation's Mr McNamara holds 11 per cent, and the Greens and Kirsty O'Connell trail with six per cent each.

Almost 20 per cent of people remained undecided.

It's a must-win election for the Berejiklian government, which will lose its working majority unless the Nationals hold the seat.

The government is already in formal minority, having to rely on the votes of two former government ministers sidelined by misconduct allegations.

Disability Services Minister Gareth Ward last week moved to the crossbench over sexual assault allegations. He joined former Sports Minister John Sidoti, who is benched amid a corruption inquiry.

Both men deny the allegations against them.

By-elections have historically proved difficult for sitting governments to win, so the poll is also an important test for Labor Leader Jodi McKay.

Her position has been under pressure this year and a failure to win could push her leadership to the brink.

Australian Associated Press