QLDC corporate and regulatory general manager Roger Taylor said yesterday the council was notified of the deterioration last week and would carry out a detailed inspection of the bridge as soon as scaffolding was erected in the Shotover River.
Mr Taylor said Downer EDI contacted GHD Consultants last week after a river user looked up and noticed "some flaking" of concrete from the underside of the bridge, understood to have been built in 1875.
It was important for the council to "understand that better" and determine the cause of the flaking.
"It was first noticed late last week. We were able to get some people out there [yesterday] to get as close a look as we can."
He said it was possible a slow leak from a water main beneath the bridge, repaired last winter, had caused superficial stress to the concrete.
"The spring thaw and wet weather may have also contributed to the deterioration of the concrete."
The ongoing operation of the bridge was "not in question", but it was important to get a full understanding of the issue.
"The bridge itself is scheduled for a full structural assessment this year, which we will now bring forward to February, but at this stage we have no reason to be concerned, given there are no indications of stress in any of the key sections of the bridge, namely the transoms, the main beams and the arch. The bridge remains sound."
Mr Taylor said the council was taking a "very precautionary approach" and erecting a tarpaulin to cover the centre span of the bridge, expected to be in place this afternoon.
The scaffolding would allow engineers to conduct a close inspection beneath the bridge, but traffic flow was not expected to be greatly affected.
The only traffic beneath the bridge was commercial operator Shotover Jet and the company had been told it could continue normal operations.
Once the council had a clearer understanding of the situation, repairs would need to be done "immediately".
