Vehicle owners have been told not to use the towbars of more than 800 heavy vehicles because of serious safety concerns

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Investigators have discovered that 61 out of the 62 truck towbars they have so far inspected are not fit for purpose and might fail while on the road.

The Transport Agency today responded by revoking the certification for the towbars fitted to 802 heavy vehicles due to "serious safety concerns".

The towbars are on a range of vans and medium-sized trucks, as well some buses and motor homes.

"We are taking immediate action to revoke the certifications for all towbars certified by Peter Wastney Engineering...in order to address the serious safety risks involved," the agency's operational standards manager Craig Basher said.

The revocation notices go out to the owners today. The towbars need to be removed, replaced, or inspected and recertified.

"We understand what this action means and the potential disruption it may cause, and this is not a decision we have taken lightly," Mr Basher said this afternoon.

"Given the evidence to hand we believe it is imperative to take this action to protect the safety of road users."

Visual inspections by specialist certifiers since the publication of a safety notice in February showed that 61 of 62 towbars inspected were not fit for purpose and could potentially fail while in use on the road.

An independent engineering review established that some towing connections were not adequately designed for the loads to which they had been certified by Peter Wastney Engineering.

"To lessen the impact on owners, we have issued exemptions to allow the vehicles to continue to operate on the road in the interim, provided the towing connections are not used."

Most of the truck trailers are in the top of the South Island.

Mr Wastney operated from near Nelson. He has been suspended as a heavy vehicle certifying engineer since shortly after a truck-trailer failure last August first triggered alarms and a Transport Agency investigation. He has since surrendered his certification authority.

A visual inspection of 23 truck-trailer drawbeams and drawbars have raised concerns, with at least one drawbar requiring immediate repair.

There are 700 of so drawbeams and drawbars that Mr Wastney certified - a decision on those is expected later this week.