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Thousands remain without power in Sydney's north after a fire in the basement of an electrical substation about midday on Wednesday.
About 26,000 homes and businesses in the upper north shore and Hills districts lost power on Wednesday, blacking out traffic lights and forcing businesses to shut.
While power was gradually restored throughout the day and night to 23,000 customers in areas including Asquith, Mount Colah and parts of Hornsby, on Thursday morning Ausgrid said it had no estimated repair time for the remaining properties as the repair work was complex.
The outage forced the Hornsby RSL Club and local Event Cinemas to close, and it blacked out about 30 traffic lights in the upper north shore.
© SuppliedA power outage is still affecting thousands of customers in Sydney's upper north shore.
On Thursday morning, parts of Wahroonga, Normanhurst, Waitara, Hornsby and Hornsby Heights remained without power.
In replies to customers frustrated by the delay, Ausgrid said crews had worked through the night to repair the problem, and it hoped to have "the remaining customers restored as soon as possible".
"Restoration is taking longer than first anticipated due to the extent of the fire damage to the Hornsby substation," Ausgrid tweeted.
"At this stage we are unable to provide an accurate restoration time."
On Wednesday it took hours for Ausgrid crews to assess the full extent of the damage.
About 2.45pm, Ausgrid tweeted crews were "assessing the level of the damage before commencing work to restore power safely." Crews were still assessing the damage at 4.20pm, according to another tweet.
"As soon as extent of damage is understood, all available crews will be deployed to safely restore power as quickly as possible," Ausgrid said.
With Jenny Noyes and Ben Weir