Fonterra's had to stand down all its truck drivers above a certain weight.

The company learned from an inquiry that its truck seats are only safe for people who weigh less than 150 kilograms.

Fonterra transport director Barry McColl said they've had to re-assign all their drivers to other positions in the company until they lose the weight.

He said they're not going to second-guess the manufacturer's safety recommendation.

"If a manufacturer tells you that's what it's rated to and beyond that the seat won't necessarily perform to standard, we're not truck engineers we can't second-guess what that is."