A 50-year-old truck driver crushed and lacerated by metal beams while unloading his vehicle at a Hamilton factory remains in a serious but stable condition at Waikato Hospital this morning.

Emergency services were called to Perry's Metal Protection on Manchester St in Hamilton at 1.55pm yesterday and transported the man to hospital in a critical condition.

However, his condition had slightly improved and he was today in a serious but stable condition in the high dependency unit.

WorkSafe NZ is investigating the incident.

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Workers from neighbouring businesses told the New Zealand Herald a load of steel was being removed from the truck when it became loose and fell, piercing the man's arm, leg and torso.

One man said he heard "an almighty crash" before running out and seeing part of the man's load of steel beams had fallen off the truck.

The truck driver's co-workers were around him frantically trying to free him.

Perry's Metal manager Duane Baguley said he was working with WorkSafe and the truck driver's employer, Les Harrison Transport.

He said nothing like this had happened at the factory before.

"We don't want to have anyone - whether it's our staff or anyone coming on to our site - having any chance of being hurt."

Mr Baguley said workers who saw the incident were being interviewed to figure out how it had occurred.

Prime Minister John Key opened Perry's Metal Protection's new galvanising facility in Hamilton in 2013.

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It is the largest galvanising facility in New Zealand, with the capacity of dipping items up to 18 metres long, enabling the company to offer increased capability along with the capacity to meet their customer's current and future demands.