The union is and Alliance are trying to determine why the man was working alone in part of the Smithfield plant.

A man is believed to have had his hand cut off in a workplace incident at a Timaru meat processing plant.

WorkSafe is investigating after an employee was seriously injured at the Alliance Smithfield plant in Timaru on Friday.

Confirming the details, Meatworkers Union Canterbury branch secretary Bill Watt said the man, aged in his 40s, started work at the plant at the beginning of the week.

He was working on his own, in the renderables area of the plant, at the time of the incident, Watt said.

"It appears his hand has been severed at the wrist.

"It's a shocking thing, it's life changing."

The man was released from hospital on Tuesday night.

Watt spoke to the man's partner on Wednesday, who said they were "shocked" by the incident.

"She said the employee himself was very sore, he's shocked and depressed," Watt said.

The union was working closely with Alliance, and was trying to determine why the man was working alone, Watt said.

It appeared the man got his hand caught in an auger, but details about the incident were unclear.

A St John spokesman said a man in his 40s was taken to Timaru Hospital's emergency department with serious injuries at 10.27am on Friday.

WorkSafe has since confirmed it was notified of the incident and was investigating.

"We are unable to provide any further information now that an investigation is underway," a spokeswoman said.

In an emailed statement, Alliance Group processing general manager Kerry Stevens confirmed an employee was injured on March 17 and was taken to hospital.

"We've launched our own inquiry and we're co-operating fully with WorkSafe and their investigation," he said.

"We're supporting the employee, his family and colleagues."

Stevens said he was "not in a position to comment" on the nature of the man's injuries.

A request to WorkSafe for the number of incidents at the Alliance Smithfield plant over the past three years had to be processed as an Official Information Act request, a WorkSafe spokeswoman said.