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Wear and tear on Auckland Harbour Bridge is now so bad that trucks may have to be banned on the clip-ons for good, even after a $45 million upgrade that was supposed to make the link safe for the next 30 years.

Official reports suggest that if trucks are allowed back on the clip-ons - which an earlier report said were at risk of "catastrophic failure" - those lanes might only last 10 more years.

Trucks were banned from crossing the outer lanes in 2007 and urgent repair work ordered after engineering reports revealed the risk of damage from heavy traffic could "no longer be accepted".

Transit said the work would meet current standards to carry peak traffic loads, as well as meet extra demand generated by the Rugby World Cup 2011. But new engineering reports show the transport lifeline for New Zealand could not cope with unrestricted numbers of heavy trucks or a dedicated cycleway.

Even once the $45m upgrade is completed next year, the New Zealand Transport Agency says the steel box-girders that support the outer lanes could need to be replaced - an action that would cause an "unacceptable" impact on the Auckland economy.

The news accelerates the impetus for construction to begin on a third harbour crossing - underground tunnels costing up to $4 billion - as well as other major roading projects.

Transport Minister Steven Joyce is expected to announce that several infrastructure projects will start sooner than planned - which may include the Waterview connection to complete the western motorway to bypass Auckland.

The lifespan of the Auckland Harbour Bridge was calculated by NZTA engineers exploring the possibility of adding a dedicated cycleway to the clip-on lanes.

Last month, the NZTA (a merger of Transit and Land Transport New Zealand) told Auckland politicians and cycle campaigners that the extra weight made the proposed cycleway impossible.

A presentation to the meeting showed that if the ban on trucks was lifted, the clip-on lanes would last only 10 to 20 years.

With unrestricted traffic and the added weight of a cycleway, the safe load capacity would be immediately breached and the lanes would be unsafe to use.

Christine Rose, chair of the Auckland Regional Council transport committee, said Transit officials had earlier assured her in writing that adding a cycleway was possible, even with unrestricted traffic.

The about-turn raised the possibility that the clip-ons were in worse shape than first thought. "We had never heard of this before," she said. "Transit told us the strengthening work would future-proof the bridge for walking and cycling and prolong the life of the bridge.

"All of a sudden, to be told by NZTA officials that: 'Oh no, we can't accommodate walking and cycling, the bridge is structurally worse off than we thought'. It was a bolt out of the blue."

If the traffic restrictions stay in place, the bridge clip-ons would be safe to use for between 25 to 40 years - as long as no cycleway is built.

Wayne McDonald, NZTA Auckland regional director, said that with "prudent and careful" management the clip-ons would last at least 30 years.

The extension bridges were a sister design to Westgate Bridge in Melbourne, built at the same time in 1969, but which collapsed during construction.

"That was a warning that our designers had not got safety margins high enough. So that meant a lot of care and attention to the clip-ons here," McDonald said. "Safety for us is totally paramount."

McDonald said the clip-on lanes were not designed to carry a growing number of heavy trucks, one of which causes as much damage to the structure as 10,000 cars.

"It's like a piece of No 8 fencing wire. If you bend a piece of wire a number of times, eventually it will snap. We're never going to let that happen."

Before the $45m upgrade is completed next year, McDonald said a decision would be made whether to allow trucks back on the clip-ons.