Mr Sanderson was helping to contain a five-hectare bushfire at Stanford Merthyr on Thursday evening. 

The 48-year-old's fellow firefighters provided medical assistance, including with a defibrillator, until paramedics arrived at the scene. 

He was taken to John Hunter Hospital but police confirmed the man couldn't be revived. 

NSW RFS Commissioner Shane Fitzsimmons said Mr Sanderson's death was a tragic reminder about the dangers members face.

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

"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."

More to come