Wild weather is causing havoc across Sydney with blackouts, flight delays and a large tree crashing onto a classroom roof of an inner-city school.

Emergency crews are working to remove the tree at Darlinghurst Public School in Potts Point on Monday afternoon.

Lucky escape at Sydney’s Darlinghurst Primary School after tree falls pix: @John__Donegan http://t.co/Z7uGhikt3e pic.twitter.com/FIEG8znWna

— ABC News Sydney (@abcnewsSydney) August 18, 2014

No injuries have been reported but there have been evacuations, Fire & Rescue NSW says.

Its crews have responded to more than 200 incidents across the Sydney and Illawarra regions, mostly fallen trees and powerlines and automated fire alarms.

The NSW State Emergency Service has received more than 550 calls in the past 24 hours, mostly for leaking and damaged roofs.

The stormy weather has also caused disruptions at Sydney Airport.

Domestic and international flights departing Sydney face delays of up to an hour due to 80 km/h winds, a Sydney Airport spokeswoman says.

A severe weather warning for damaging winds and heavy rain remains in place for the Sydney metropolitan area, Hunter, Illawarra and South Coast regions.

The weather bureau says the low-pressure system is not expected to ease until Tuesday.

Meanwhile, many areas have received their best rains in months.

In the 24 hours to 9am on Monday, Porters Creek, inland from Ulladulla on the south coast, had 197mm of rain.

Further south, waves of up to eight metres were pounding Batemans Bay.

Sydney can expect waves of four to five metres on Tuesday.

Thousands of homes across the region have lost power.

Crews are working to restore power to about 4000 homes across the Illawarra and parts of Sydney.