Power has been restored to 31,000 homes and businesses affected by a blackout on Sydney's northern beaches on Tuesday afternoon.

About 4.30pm, Ausgrid tweeted that emergency crews were working to restore power to homes and businesses in Avalon, Mona Vale, Newport, Warriewood, Palm Beach, North Narrabeen, Elanora Heights, and surrounding suburbs.

It is understood the blackout was caused by an electrical transformer failing due to increased load on the grid.

It is not yet known whether the transformer is owned by Ausgrid or TransGrid.