Police are investigating how a man suffered fatal injuries at Cessnock Waste Management Centre after closing time on Sunday.
A post-mortem was expected to take place on Tuesday on the body of 54-year-old Shane Mears, who suffered what a senior detective described as " multiple, significant injuries".
"We believe those injuries have led to the man's death," Hunter Valley Police District crime manager, Detective Inspector Matt Zimmer said.
"Until we know the exact circumstances of what's happened and the cause of death, we are treating the death as suspicious."
Police are investigating the possibility Mr Mears and a friend sneaked into Cessnock Waste Management Centre via a bush track.
The friend - a 46-year-old man from Cessnock - has been helping police with their inquiries.
He had not been arrested and ACM understands he was not a suspect at the time of publication.
Investigators believe the pair came across other people at the tip before Mr Mears suffered his fatal injuries.
"We do believe there were other people in the vicinity of where the deceased male was, shortly prior to his death," Inspector Zimmer said.
"Those people are unknown to us at this stage."
Police were called to the Old Maitland Road tip just after 5pm on Sunday after reports a man had been critically injured.
There, they found the 54-year-old - who died at the scene.
Detectives set up Strike Force Nargal to investigate the incident.
A forensic examination of the site began on Sunday night and continued into Monday, with police focusing on what appeared to be two separate areas separated by a large mound of scrap metal.
"It's quite a remote area, where the deceased was located," Inspector Zimmer said.
"There's a lot of bushland surrounding that particular area."
Mr Mears was previously known to police and Inspector Zimmer said the Cessnock man's "background and associations" would be looked into as part of the investigation.
He said police were working to identify the other people who were inside the grounds of the dump when the 54-year-old was injured, and he called on anyone with information to come forward.
"We are still in the early stages of this investigation," he said.
"We are still trying to establish the circumstances surrounding the man's death. We are asking anyone with information to contact [police]."
Anyone with information can contact Crime Stoppers on 1800 333 000.
EARLIER REPORT: Monday, July 6, 11.10am
Strike Force Nargal has been formed to investigate the death of a man at a rubbish tip at Cessnock on Sunday.
Emergency services were called to the waste management facility on Old Maitland Road just after 5pm Sunday, after the 54-year-old Cessnock man was found critically injured in a remote part of the facility. He died at the scene.
Officers from Hunter Valley Police District established a crime scene and detectives are now investigating the circumstances of the man's death.
As this is an unexplained death, police are currently treating it as suspicious.
Hunter Valley Police District crime manager, Detective Inspector Matt Zimmer addressed the media at Cessnock Police Station on Monday morning.
Detective Inspector Zimmer said the man was found in an isolated part of the waste management facility, with "multiple significant injuries".
He said a 46-year-old Cessnock man - who was an associate of the deceased - is assisting police with their inquiries, and that police believe a number of other people were in the vicinity.
Anyone who was at the tip between 4.30pm and 5pm Sunday, or has any information about this incident, should contact Cessnock Police on 4991 0199 or Crime Stoppers on 1800 333 000 or https://nsw.crimestoppers.com.au.
A Cessnock City Council spokesperson has confirmed the waste management centre is open as usual on Monday.