KEY POINTS:
Trucks will be banned from the outside "clip-on" lanes of the harbour bridge, casting doubt over its future as Auckland's transport lifeline.
Transit New Zealand will move immediately to stop trucks using either of the outside lanes, a measure it says is to make the bridge last longer.
Buses are not included in the ban, which will apply to any vehicle heavier than 4.5 tonnes - essentially anything that looks like a truck.
Truck companies have been asked to stay on the centre lanes, although it will take about two months to amend a bylaw enforcing the restriction.
Transit chief executive Rick van Barneveld said Transit was not keeping any problems with the bridge from the public.
He said trucks caused "fatigue stress" to the clip-ons, and reducing that would mean the additions would last at least a further 40 years. He believed the main bridge structure was sound.
"Nothing would have happened to the bridge. It was just going to keep costing us more and more in maintenance."
Once the bylaw was amended, Mr van Barneveld said the change would be advertised and signs put up on the bridge.
Police would enforce the ban, although "we're not going to be pinging anyone for a while yet".
Associate Professor John Butterworth, of Auckland University's civil engineering department, said the clip-on lanes were operating close to their limits.
He said the steel box-girder construction of the clip-ons tended to flex and crack and had to be regularly inspected.
He said the lanes could be kept up to strength or even improved by cutting out and replacing parts.
Keeping heavy trucks off the outside lanes would help.
Auckland Mayor Dick Hubbard said last night the ban was "a big wake-up call". He had not known the restrictions were to be introduced.
Mr Hubbard said he had been concerned about the city's "dependence on a single ribbon of steel" which was vulnerable to structural damage to the clip-ons, a terrorist attack or a container ship crashing into it.
"It goes without saying that if anything happened to that bridge Auckland would be paralysed.
"Even the need to replace one clip-on would result in absolute chaos."
Mr Hubbard said it was time to decide on another harbour crossing - a project that could take up to seven years to plan and many more to build.
Mr Hubbard favoured a tunnel under the harbour on the eastern side of the bridge. Another option was a bridge on the western side that ran through to Northcote.
"I don't mean to appear alarmist but this means something needs to be done."
The bridge opened in 1959 and the clip-ons were added in 1969.
Restrictions were also placed on the outer lanes in 1985 after the discovery of 40 cracks in the bridge's steelwork.
Transit said the number of trucks crossing the bridge had increased markedly since.
Acting Transport Minister Harry Duynhoven said Transit had assured him there were no problems with the bridge and that the decision was prudent to limit the wear and tear.
North Shore Mayor George Wood could not be reached last night.
Auckland Regional Council chairman Mike Lee said it would be "unwise to sensationalise the move".
Road Transport Association spokesman Chris Carr said the organisation - which covered up to 20,000 trucks in Auckland - supported the Transit decision.
Mr Carr, from transport company Carr-Haslam, did not expect any disruption to drivers as it was easier to stick to the middle lanes of the bridge if they were going through Auckland.
About 300 million tonnes of freight were moved through Auckland each year, he said, and the public needed to remember there was "no public transport solution for what trucks do".
"The bridge isn't big enough to cope any more.
"And everyone knows that a 50-year-old needs more maintenance than a 10-year-old."
- additional reporting Angela Gregory
The clip-ons: A Brief History
Growth on the North Shore and increasing traffic led to the additions of two-lane box girder clip-on sections to each side of the bridge in 1969, doubling the number of lanes from four to eight.
These were made in Japan, which led to the nickname "Nippon clip-ons". A week before their official opening, cracks were discovered in the welds, prompting a Ministry of Works investigation.
The clip-ons had an expected lifetime of 50 years and were expected to need replacing by 2016.
Ministry of Works staff quietly repaired a girder that had buckled several centimetres out of line in 1970. The problem and its repair in 1971 was made public in 1982.
The discovery of 40 cracks - up to 60cm long and 2mm wide - on the bridge's steelworks in 1985 prompted restrictions on trucks and buses using the outer lanes.
