Ministry of Business, Innovation and Employment health and safety manager John Howard said the maintenance was not carried out according to procedure.
"An interlock device would have prevented this accident by stopping the chipper hood from being opened to expose the blades when the drum was still rotating," he said.
"Even though the principles of machine guarding are well known, people are still seriously injured and killed because machines are poorly guarded or not guarded at all.
"It is essential that all those with duties under the Health and Safety in Employment Act understand the hazards associated with the use of machinery in the workplace."