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