Investigators are now trying to work how a mixer was turned on with people inside, killing one a man at a Sydney factory on Thursday morning.

The 42-year-old contractor had been cleaning the ink vat at DIC headquarters in Auburn when he and two other men became trapped and injured, sparking a large response from emergency services.

Two other men, aged 28 and 29, were rescued by emergency services and taken to Westmead Hospital with leg injuries.

SafeWork NSW investigators returned to the site on Friday to work out exactly what happened, a spokesman said.

"Initial inquiries indicate the mixer activated while two workers were undertaking maintenance on the tank. A third worker, aged 28, is reported to have gone to their assistance and been injured," the spokesman said.

"A 29-year-old worker that was undertaking maintenance on the tank was freed by emergency services while the other worker, aged 42, was trapped by the mixer's blade and passed away from injuries he sustained."

SafeWork engineering experts were inspecting the mixer to work out the cause of the incident, and NSW Police were also preparing a report for the coroner.

NSW Minister for Innovation and Better Regulation Matt Kean said the death was "totally unacceptable".

"No worker should go to work and be injured on the job, let alone lose their life," he told Nine News.

"We will do whatever is necessary to get to the bottom of what's gone on, and hold those responsible to account."

On Thursday, NSW Ambulance Superintendent Paul Turner said the death was a tragedy.

"When someone deteriorates clinically it's always hard for all emergency workers," he told Fairfax Media.

"They build rapport with the patient; to have him deteriorate and lose his life can be quite distressing."

Superintendent Turner said ink slush at the bottom of the vat made the rescue operation complex.

"It makes it quite difficult to access the patients ... because you're in a really dirty environment; obviously that hampered efforts," he said.

The vat was between five and eight metres high and emergency workers used a manhole at the bottom to access the patients, he said.

Superintendent Turner said there had been "great co-operation" between the different emergency services at the scene.

"People were working extremely hard to get all the men out."