The Christchurch earthquake had Dunedin City Council staff checking for fresh cracks at the Ross Creek Reservoir dam.

The dam was patched in June, after large cracks were discovered across the face of the 143-year-old earth wall.

Council water production manager Gerard McCombie yesterday said he was back at the dam by 6.45am on Saturday - just over two hours after the 7.1 magnitude earthquake devastated parts of Christchurch - but found no signs of fresh damage.

The dam was not the only one checked. Mr McCombie said it was standard practice to visit dams around the city after an earthquake. No signs of damage had been detected at any of them.

Mr McCombie returned to the Ross Creek dam again yesterday to carry out further checks, and again found no reason for alarm.

"I was reasonably confident but ... it's an old structure and you have just got to keep an eye on it."

Mr McCombie said the possibility of a large earthquake striking the area had been considered when Opus International staff were asked to assess the dam's stability, after the discovery of its cracks on June 3.

Opus staff had decided the dam was safe despite the cracks, which were patched up until long-term decisions about the dam's future could be considered by city councillors.

Opus International technical services manager Ian Walsh, of Dunedin, said there was "a good chance" the dam would have withstood a 7.1 magnitude earthquake, even if it were centred on Dunedin.

The cracking in May was the result of saturated ground, which had since begun to dry.

Further damage would depend on a combination of groundwater inundation and the size of any earthquake.

- chris.morris@odt.co.nz