More than 4000 homes without power on Sydney's Northern Beaches

More than 4000 homes and businesses in Manly, on Sydney's Northern Beaches, were without power most of Sunday since the early hours of the morning.

Ausgrid said an underground fault was to blame and promised to have it fixed before midnight.

Thousands of properties were cut off, mostly on Manly's eastern hill, from 4am.

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Bear Cottage, a hospice for children, was forced to use back-up power.

More than 105 rooms at Manly's Q Station hotel, cafe and restaurant were in the dark, forcing a number of guests to check-out early.

At Manly Skiff Club, 300 lunch reservations were cancelled and bookings to watch the State of Origin were put on standby.

According to its website, Ausgrid said it was aiming to get the power back by 11pm.

The underground fault is at the North Head substation.