Former Upper Hunter MP for the Nationals, Michael Johnsen, resigned from Parliament last month over sexual assault allegations, which he has denied.

Dr McGirr said an Upper Hunter loss next month could leave the government relying on the Speaker of the Parliament's casting vote to pass legislation.

"You don't want anyone to miss a vote, it's tight and it just makes the situation the government's already in tighter," Dr McGirr said.

"[The independent MPs] have indicated that we will maintain confidence of supply unless there is serious mismanagement or corruption, so even though the numbers will get tighter, I think the government will be OK."

Dr McGirr dismissed comparisons between Upper Hunter and Wagga's byelection, saying he did not expect a repeat of the swing against the government that helped him win in 2018.

"Obviously both byelections were precipitated by a scandal, so that's a similarity, but I think the situations are quite different," he said.

"[Upper Hunter] is already marginal and in the Wagga byelection, people could vote against the government and send a message and they would still be certain that the government would continue as it had a reasonable majority.

"In this circumstance, because the numbers are tighter, I think people will think more carefully about voting against the government."

In other news

Charles Sturt University politics professor Dominic O'Sullivan said the Coalition could still count on enough votes in Parliament regardless of the byelection outcome.

"NSW seems to be handling the pandemic reasonably well ... this is probably not a good time for an independent like Joe McGirr to find a reason to bring the government down," he said.

Professor O'Sullivan also agreed that Upper Hunter would be different to Wagga's byelection.

"In Wagga there was an obvious alternative independent candidate and somebody who already had a profile and strong support base and was able to exploit the dissatisfaction with the Liberal Party," he said.

NSW Premier Gladys Berejiklian said she had "no doubt" that the government will lose the seat because voters "will reflect what they're feeling".

Labor leader Jodi McKay responded by stating it would "take a miracle for the Nats to lose the seat they've held for 90 years".

One Nation, Greens and the Shooters party are also expected to run.

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