Ian Pullen was found dead beside a road near Singleton just after 5.30am on September 29, 2018, a day after he arrived to help the NSW Rural Fire Service prepare for the bushfire season.

The mystery - the 43-year-old was the victim of an apparent hit-and-run - prompted a two-year investigation involving multiple appeals for information and a $350,000 reward posted by the NSW government.

Investigators on Monday charged a third person in relation to Mr Pullen's death, arresting 21-year-old Lilli-Jane Kaye Sales at a Whittingham home.

She was charged with being an accessory after the fact to murder and faced Muswellbrook Local Court on Monday, where she was granted bail.

Ms Sales will face Singleton Local Court on January 28.

Detectives made their first arrest in the case last month, charging 29-year-old Singleton man Joshua Knight with murder, dangerous driving occasioning death and failing to stop after a vehicle's impact had caused a death.

Less than a week later, police charged 30-year-old Nicole Mason with murder.

Police allege the pair were in a ute being driven by Mr Knight when it hit Mr Pullen, before they left the scene, returned and struck the pilot to the head with an unspecified object.

Investigators believe Mr Pullen was still alive when the pair got back to the scene.

Mr Pullen's wife Vicki told the Newcastle Herald it meant "so much" to her family that such time and resources were continuing to be put into finding the people responsible for her husband's death and holding them accountable.

"Another arrest shows how determined the NSW Police are to bring justice for Ian," she said on Tuesday.

"I thought I had been holding up well until this latest arrest. Pretty raw emotions are coming out today.

"The person and life I knew has been destroyed."

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