Three people have already been charged over the murder of Brett Watson, 41, whose body was found by a motorcyclist in Greengrove, near Wisemans Ferry, in October.

But police have revealed they believe a fourth person was also involved in the alleged bashing murder of the St Marys man.

They are also trying to track the movements of Mr Watson's car, a 1999-model white Ford Mondeo with registration plates BAX47A, that was seen in the Minto area on October 20, the day of his alleged murder.

A motorcyclist discovered Mr Watson's body in secluded bush near the intersection of Mangrove Creek Road and Wisemans Ferry Road in Greengrove, a few kilometres from a yoga retreat, about 7.30am on October 21.

Police will allege in court that Mr Watson was killed at a unit on Morris Street in St Marys, before his body was dumped in Greengrove.

Detectives set up Strike Force Pilliga to investigate Mr Watson's death.

A short time after the body was found, police arrested a man, 35, and a woman, 36, and charged them with Mr Watson's murder. The pair are due to appear in Penrith Local Court on Friday.

The week after those arrests, police arrested a 42-year-old man in Kilaben Bay, near Newcastle, and charged him with being an accessory after the fact to murder.

He was granted conditional bail and is also due to appear in Penrith Local Court on Friday.

"Investigators have information that a fourth person was involved in the alleged murder and are seeking public assistance to identify this person," police said in a statement.

Anyone with information about the fourth person, or who saw Mr Watson's car around the time of the murder, has been urged to contact Crime Stoppers on 1800 333 000.