Sydney's Upper North Shore and Hills District have been hit by a blackout with about 26,000 homes and businesses affected.

Ausgrid said power supply was interrupted about midday affecting parts of Hornsby, Waitara, Asquith, Wahroonga, Galston and Normanhurst.

Ausgrid tweeted a fire in the cable basement of the Hornsby zone substation had caused the power failure but couldn't say when power would be restored due to the complexity of the repairs required.

Spokeswoman Michelle Stone apologised for the power failure.

"Emergency crews need to isolate the fault before they can begin to restore the power safely," she said.

"As soon as we know the ... restoration time we'll be posting that on our website."

The Transport Management Centre said numerous sets of traffic lights were blacked out and they advised motorists to reduce their speed and exercise caution.

The Westfield in Hornsby is affected, as well as other businesses including car dealers, gyms, hair salons and fast-food restaurants who have warned electricity may not be restored until after 2:00pm.

It is the second major outage in Sydney in two weeks, after the eastern suburbs were brought to a standstill leaving 45,000 customers without electricity.

Crews are working on repairing an underground cable fault.