A Rural Fire Service (RFS) volunteer has passed away while fighting a bushfire in the New South Wales Hunter Valley.

The man was working to contain a fire at Stanford Merthyr near the town of Cessnock when he suffered a suspected heart attack, the RFS said.

RFS Commissioner Shane Fitzsimmons said the man's death was a tragic reminder about the dangers faced by firefighters.

"This is a terrible tragedy, where somebody who has gone to assist the community in its time of need, has paid the ultimate sacrifice," he said in a statement.

"Our thoughts, prayers and condolences are with the member's family, fellow brigade members, and colleagues.

"We are offering every assistance possible to the volunteer's family and his colleagues."

Fire crews had been working to contain a spot fire that jumped the Hunter Expressway at Buchanan, near Stanford Merthyr.

Westbound lanes of the highway were earlier closed as 70 firefighters and water-bombing helicopters battled the large blaze burning in bushland.

In a statement, the NSW Minister for Emergency Services, David Elliott, expressed his condolences.

"I am deeply saddened by the death of a NSW Rural Fire Service volunteer in the Hunter region," Mr Danks wrote.

"The volunteer, like all our emergency service workers, was selflessly dedicating his time to the protection of the community.

"It is a timely reminder of the dangers faced by our emergency service workers every day."