A fire in the cable basement of the Hornsby zone electrical substation has caused a mass power outage leaving thousands of properties without power across Sydney's upper north shore and Hills district, with "no estimated restoration time" on the horizon.

Supply was interrupted at midday on Wednesday to about 26,000 properties across parts of Hornsby, Waitara, Asquith, Hornsby Heights, Wahroonga, Galston, Mount Colah and Normanhurst, Ausgrid said.

The outage has disrupted activities in the area's schools, and caused shops and other businesses to close.

In a Facebook post, Westfield Hornsby said Woolworths and other fresh food retailers were still trading, although other shops in the centre have closed.