can have a failure. However, those without the ability to visually inspect the skidplate structure are most susceptible to unforeseen failures. These can include refrigerated trailers, tankers, bulk transporters, etcetera," the Transport Agency said.

MaxiTRANS said it was working closely with the Transport Agency.

MaxiTRANS, which is headquartered in Victoria, Australia, said that while it has been specifically named by the agency, the safety alert applies to other brands of refrigerated semi-trailers too.

It said it endorses and reiterates the importance of all operators adhering to a regular maintenance and inspections.

Mr Wastney of Peter Wastney Engineering told RNZ it was not a public safety issue then hung up, twice.

He was suspended as a certifier in August, and as a chartered professional engineer last week.

The August and February failures were related tohis work.

"Independent reviews of those drawbeams and drawbars have identified that they were not adequately designed for the loads to which they had been certified," the Agency said.

Kelvin Barclay, who heads the Heavy Vehicle Certifying Engineers group at Engineering New Zealand, said the two alerts were unprecedented in a certification industry set up only in the late 1980s.

"What happens when there's an uncoupling between the truck and a trailer is the brakes on the trailer automatically come on, but obviously it takes some time for it to come to a stop and in that time it's basically out of control, so could go anywhere," Mr Barclay said.

Steel fatigue was the underlying issue and the cracks just a symptom of that, he said. Trailers were re-certified after 10 years