Transit is confident the worst-case scenario flagged in the report will never happen.
The Auckland Harbour Bridge could be more vulnerable to extreme winds than previously thought.
A report by the Beca engineering consultancy says some of the structures supporting the clip-ons "were found not to satisfy standards in maximum winds" when added to maximum traffic loadings.
Wind effects could be
15 per cent greater than thought.
The report said the clip-ons were at risk of "catastrophic failure" if heavy trucks were forced to a crawl or halt in both lanes of each.
"If the level of load in this assessment, representing a traffic jam situation with maximum heavy vehicle concentrations at the centre of span two [the central span] in both lanes, were to occur there would be a risk of catastrophic failure of the deck."
Transit explains it was a worst-case scenario, of a "very large number of heavy trucks stopped on the central spans" by a traffic jam further up the motorway.
The agency concluded there was no real prospect of a such a traffic jam, but has restricted trucks, to be safe.
Buses were not considered a threat to bridge safety.
Transit chief executive Rick van Barneveld last night said wind tunnel testing in Melbourne would help pinpoint any other weaknesses in the clip-ons. A $45 million repair programme has been fast-tracked.
"If this testing shows further opportunities for strengthening the bridge, taking advantage of the latest technological advancements, it would make sense to include this work at the same time as the already scheduled strengthening work for the clip-ons," he said.
