The Shooters, Fishers and Farmers Party (SFF) are likely to be the only party that has a female candidate in the Upper Hunter by-election.
President of the Singleton Business Chamber Sue Gilroy will this morning be introduced as the candidate for the SFF at the launch of the minority party's campaign to take the seat from the Nationals.
The Nationals have held the seat for 90 years and if they lose it, the Berejiklian government will be in minority.
Ms Gilroy has been at the helm of the Business Chamber for three and a half years, and runs her own business as a personal business mentor.
The Singleton local is also a registered nurse who has worked in the coal mining industry as an occupational health nurse and in rehabilitation services for injured workers.
The Nationals have made it clear they are courting the mining vote, launching their campaign last Thursday at the Ravensworth open-cut mine near Singleton.
Nationals candidate David Layzell, 45, is a construction manager from Dungog.
The ABC understands the Nationals would have preferred a woman - the first female mayor of Singleton Sue Moore - but Mr Layzell won the grassroots preselection in which 23 local National members voted.
Premier Gladys Berejiklian has already said it would be a "miracle" if the government retained the seat, while Deputy Premier and Nationals leader John Barilaro also acknowledged "by-elections don't favour governments of the day".
The SFF candidate is considered a major threat, with the Nationals already losing three seats to the party in less than five years.
In 2016, the SFF took the seat of Orange from the Nationals in a by-election after a swing of almost 22 per cent. The Nationals also lost the seats of Murray and Barwon to the party after the 2019 state election.
The Upper Hunter by-election was triggered by the resignation of Michael Johnsen after it was revealed he sent a series of lewd text messages to a sex worker during question time. He is also accused of rape in 2019, an allegation he strenuously denies.
The Nationals hold the seat with a slim margin of 2.6 per cent.
The margin was narrowed by Labor in the 2015 election.
Labor is expected to announce its candidate early this week, who is likely to be local mining union official Jeff Drayton.
One Nation, which performed well in the Hunter region in the 2019 federal election, is yet to reveal its candidate.
The Upper Hunter is the state's biggest mining electorate, taking in Singleton and Muswellbrook, but also has large rural areas including Scone, Murrurundi, Dungog and Gloucester.
The by-election will be held on May 22.