Electricity has been restored to more than 20,000 Ausgrid customers after storms lashed Sydney but parts of the city are facing another powerless night.

Up to 29,000 Ausgrid customers remain without power on Wednesday morning after severe storms battered Sydney on Tuesday and caused "extensive damage" to the city's electricity network.

The utility company said suburbs in the north of Sydney are now facing the prospect of extended outages.

"Power is still out to about 29,000 homes and businesses, mostly in Sydney's north," an Ausgrid spokesman said in a statement.

"Customers in the worst-affected areas are being advised to prepare now for another night without power overnight into Thursday."

STORM UPDATE:

Our crews have safely restored power to 23,000 customers following yesterday’s storms. Power is still out to 29,000 mostly in Sydney’s north. Customers in the worst hit areas are being advised to prepare for another possible night without power. #sydneystorms pic.twitter.com/RNcKp4BMo6 — Ausgrid (@Ausgrid) November 26, 2019

The worst-affected suburbs included Roseville, Gordon, Epping, Chatswood, Frenchs Forest and surrounding areas, Ausgrid said.

Ausgrid said extra crews were coming down from Newcastle and the Central Coast to help with repairs, with storm-related damage to be prioritised.

Over 70 crews out in the field at first light this morning following the severe storms that ripped through Sydney, Illawarra and Northern NSW yesterday #sydneystorm #Storms pic.twitter.com/wlTND0AQYl — NSW SES (@NSWSES) November 26, 2019

NSW was a "state of extremes" on Tuesday as severe storms downed trees and powerlines in Sydney, snow fell in Thredbo and other parts of the state sweltered.

Winds reached up to 104 km/h at Fort Denison, closely followed by 96km/h in Holsworthy and 95km/h in Sydney Harbour, the Bureau of Meteorology said.

Trains were stopped on Tuesday on sections of the North Shore and Central Coast and Newcastle lines, Transport for NSW said.

+ more than i saw around the north shore from #sydneystorm pic.twitter.com/DfPjuSULlV — Alivia (@aliviarh) November 26, 2019

About 51,500 homes and businesses lost power after the storms on Tuesday afternoon, Ausgrid said.

Sydneysiders have been warned to stay well clear of fallen wires and debris.