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.
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.