A co-worker found Kevin Hanson, 39, dead on Wednesday afternoon at the colliery northwest of Lithgow.
A contract technician from Fuchs Lubricants, Hanson had been working underground collecting oil samples about 8km from the mine entrance.
Police said an inspection of the immediate area found a broken high-pressure hydraulic hose, which is believed to have struck the Kurrajong Heights man when it ruptured.
A Department of Primary Industries spokeswoman said inspectors were searching for a staple lock fitting – the piece believed to have come off the high-pressure hydraulic hose, causing the accident.
Centennial Coal chief operating officer David Moult extended his condolences to the man's family and colleagues and said the company was cooperating with authorities.
“The safety of our people is the company's number one priority,” he said.
“The company extends its thoughts and prayers to the family, friends and colleagues of the deceased.”
Construction, Forestry, Mining and Energy Union southwestern district vice-president Graeme Osborne said the mine reopened on Friday but Hanson's colleagues were shocked and saddened by his death.
He said Hanson had worked for Fuchs Lubricants for 15 years and was a veteran of mines in the area.
Police, the DPI and the CFMEU are investigating the incident.