Heavy trucks should be banned completely from the Auckland Harbour Bridge clip-on lanes in high winds, according to an official engineering report - despite earlier assurances from Transit that no such measures were necessary.
Extreme winds have emerged as a potentially greater risk than previously thought by Transit, and a recent report from Beca Infrastructure recommended further tunnel wind testing in Australia.
The June report also said more strengthening work was needed to cater for potential contact between the main truss bridge and the clip-on extensions.
"It can be confirmed that heavy traffic must be restricted on the extensions in high winds.
"Under peak traffic loading, the box girder touches down on the truss bridge, requiring local strengthening of both the box girders and truss bridge."
Transit New Zealand chief executive Rick van Barneveld originally said there was no need to ban heavy traffic from the inner clip-on lanes.
Last week he told the Herald on Sunday that Transit monitors wind speed on the bridge constantly and traffic management plans limit the use of the bridge by heavy or high-sided vehicles.
"Wind speeds in excess of 120kph result in heavy vehicle restrictions and the purpose of additional wind tunnel modelling is to refine our setting of the wind level indicator," van Barneveld said.
The structural work needed on the main bridge was straightforward and minor, he said.
The revelations come as Transit New Zealand seeks to quell fears that the bridge's clip-on lanes could suffer "catastrophic failure" in extreme circumstances, as reported by the Herald on Sunday last week.
The papers, released under the Official Information Act, state that Transit officials had underestimated the traffic load on the clip-ons, which failed to meet buckling strength standards in key areas by a "large margin".
In a worst-case scenario predicted to occur just once every 2000 years - a traffic jam of fully laden trucks - the clip-ons were at risk of "catastrophic failure" of the deck.
Transit has announced a $45 million upgrade to fast-track strengthening work. It has banned trucks from the outside lane of the clip-ons, but assured motorists that the bridge is safe.
Van Barneveld said there was "absolutely no alarm", as the truck ban, traffic diversion from around the clock camera surveillance, and the safety work made the bridge "perfectly safe".
Transport Minister Annette King has accepted assurances from van Barneveld that the Auckland Harbour Bridge is safe.
She met Transit officials most weeks and had been briefed on the engineering reports but would not comment on them, a spokesman said.
King had "happily driven over the outside lane on many occasions", he said.
The new mayor of North Shore, Andrew Williams, was keen to fast-track a second harbour crossing, to be completed in the next 10 years.
Transit has invited him, Auckland Mayor John Banks, and Auckland Regional Council chairman Mike Lee to inspect the bridge this week.
"I think the bridge is safe but it's starting to have a few issues," Williams said.
"If you don't have an alternative underway, in 10 years' time, it might too late."
"Even if one clip-on was closed for six months while they fix it, the impact on the New Zealand economy would be huge."
A Transit report on a second harbour crossing is due in March. It is thought the highway builder prefers a $3 billion underground tunnel.
What the experts say
Mar 2005: Auckland Harbour Bridge Data Review
Heavy commercial vehicles increased by 15-20 per cent since 2002.
Clip-on fatigue life predictions and repair forecasts should be reassessed.
Nov 2005: AHB Annual Inspection Truss bridge has minor defects. Clip-ons in good condition with the exception of "known defects".
Jul 2006: AHB Box Girder Assessment Stage 1
Five key areas of clip-on lanes had "insufficient buckling strength".
Recommendations: Manage traffic to prevent queues of heavy vehicles.
Enforcement to prevent overweight vehicles using clip-ons.
Prevent heavy vehicles using the shoulders on extension bridges.
Nov 2006: AHB Traffic Load Update"Significant" increase in loading from most heavily loaded lane between March 2002 and March 2005.
Dec 2006: Maintenance Strategy UpdateOverall condition of bridge good. Weld cracking not of great concern.
Dec 2006: AHB Box Girder Assessment Stage 2
Several areas of clip-ons well short of adopted assessment standards. Most notable deficiencies in load-carrying capacity found at spans 2 and 3.
Increased traffic means reduced safety margins. Intervention necessary to prevent heavy traffic loads.
Implications: Midspan most critical area. Upper web panels and stiffeners had inadequate load-carrying capacity. Risk of catastrophic failure with maximum heavy vehicle concentrations at centre of span 2 in both lanes.
Feb 2007: AHB Box Girder strengthening concept design briefKey areas of box girder structure below standard and need strengthening.
June 2007: AHB Box Girder strengthening concept design reportClip-on brackets and piers did not to satisfy standards in maximum winds with maximum traffic loadings.