A high ladder fire truck had to stop on Auckland's Southern Motorway on the way to a large industrial fire in Takanini to wait for an escort because its lights and sirens were not working.

The incident yesterday is the latest in a string of mechanical failures among Fire and Emergency New Zealand's fleet of fire appliances and comes just days after one Auckland station had to rent a light truck following an engine blow out.

But Fenz said its fleet meets the needs of the public and it had taken steps to address problems including a project to introduce 36 more appliances by late next year.

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Fenz assistant area commander John Booth confirmed the lights and sirens were broken on the Auckland city aerial appliance that responded to last night's industrial fire in South Auckland.

The fire truck had to be pulled over on the motorway to await escort from another engine.

New Zealand Professional Firefighters Union Auckland Local secretary Martin Campbell said the incident was another embarrassment.

"It's just another sign of the state of the fleet that Fenz has got."

Cracks have appeared in the locker structure of Type 3 Fraser-MAN trucks, of which there are 47 nationwide. The trucks are four years old.

Of 42 inspected, 23 have cracks; 10 in Auckland, seven in Wellington, and one each in Gisborne, Tauranga, New Plymouth, Whanganui, Christchurch and Dunedin.

A Type 3 truck is a larger fire truck mostly used in cities and larger urban areas.

It follows a blacklisting of the appliances in early 2017 after firefighters were stranded inside a fire when a water pump failed.

Fenz would not confirm whether it cost the organisation $130,000 per truck to fix the issues, which also included a lack of acceleration, or that it took the total cost of each truck to $900,000.

And during the SkyCity Convention Centre fire in October only one aerial appliance with a ladder at least 30m high responded for the first two hours, with the union claiming the fire could have been put out within that time frame if more high ladder trucks were available.

The other heavy aerial appliance was being serviced and had to be recommissioned, and a third, relief heavy aerial has been off the road for a year because of mechanical issues.

Fire staff had to call in a high ladder truck from Hamilton.

At Campbell's station in Grey Lynn an 18-year-old salvage tender that carries vital equipment such as ventilation and tarpaulins broke down last week and firefighters were forced to hire a light truck because no relief appliances were available.

Campbell said it meant the temporary tender cannot travel under lights and siren and took longer to get to emergencies.

"We had to respond that rental van to a building fire in Māngere. We had to drive through city traffic at 6.30pm at normal speed.

"That white truck symbolises the state of Fenz's fleet."

Of the 227 Type 3 appliances overall, 19 per cent were between 25 and 30 years old, Fenz deputy chief executive of strategy and capability development, Russell Wood, said.

Wood said cracks were initially discovered in the Fraser-MAN Type 3 trucks during a Certificate of Fitness inspection.

"A heavy vehicle engineer has assessed the affected trucks. They have confirmed the cracks in the structural supports are not a safety issue for our crews or the public.

"As a precaution, if any cracks are found, the appliance will be taken off the road to be repaired."

The Herald understands at least 10 of the trucks are off the road in Auckland.

"We are working under urgency with our supplier to identify the cause and fix the issue across these appliances. The solution will be also be designed into all future Fraser-MAN Type 3s introduced into the fleet."

In 2011 Fenz called for tenders for the supply of cabs and chassis and MAN, a German truck manufacturer, was successful.

Once in New Zealand the trucks are fitted out and made into fire engines by Fraser Engineering.

Campbell said Fenz chose the trucks without importing prototypes first.

Wood admitted there were "some problems in the past" with the Fraser-MAN Type 3s but that the key issues had now been fixed.

He said Fenz had a project under way to acquire a new version of the Type 3 but it would take some time for those appliances to be developed, evaluated, built and delivered.

"While the project is under way, we will be introducing 36 more MAN Type 3 appliances over the next few years.

"This is so we can maintain a modern fleet and our people can continue to have the right tools and resources to meet their community's needs."

The first of those trucks is expected in late 2020.