Council resource consents manager Alan Worthington said the cables - previously unknown to the council or candidates - had been confirmed under sites in Main South Rd and Brighton Rd, in Green Island, and at Maia.
The discovery came after a volunteer working for mayoral candidate Lee Vandervis pierced an 11,000-volt cable while hammering in a steel peg to support one of Mr Vandervis' large swing signs in Quarry Rd, Mosgiel. He was not injured.
The pin damaged the cable, which later failed, cutting power to 747 consumers from East Taieri to Taieri Mouth.
An email containing updated advice from the council's planning department was sent to all candidates yesterday, warning it was "likely that there are underground services running through many of the sites for election signs".
"This may include water, sewerage, power, telecommunications and gas reticulation," it said.
Telecommunication networks were also generally much closer to the surface than power cables, and "damage to fibre optic cables would be likely to result in considerable expense for repairs", it warned.
Mr Worthington told the Otago Daily Times there was also the risk other low-voltage cables were underneath the sites.
"The advice we're giving is you need to take care and check with Delta," Mr Worthington said.
He was not sure of the voltage of the other cables confirmed under other election sites, and staff from Aurora Energy - the Dunedin City Council-owned company which owned the network - were not available for comment yesterday.
Earlier this week, an Aurora spokesman confirmed there was no warning sign at the Quarry Rd site to alert the public to the existence of the high-voltage underground cable, but Mr Worthington could not say if signs were at the other sites.
The discovery of the cables was "probably a concern", but also showed the style of election advertising signs had changed, he said.
"When the scheduled sites were created ... the intention was that people would be putting up lightweight signs with small wooden posts.
"We've probably progressed a bit beyond that."
As well as warning candidates, the council would review its rules governing the use of the sites in time for the next election, he said.
"That has made us have a close look at these and take some steps to ensure candidates are fully informed of care they should take when putting signs up."
However, he did not think Aurora or Delta Utilities Services - another council-owned company, which managed the network for Aurora - would be required to make changes.
"The services, I'm sure, have been put in in accordance with their requirements. It's quite common to have services underground and people just need to take care."
