NSW residents have been told to brace for more wild weather with gale-force winds and the risk of large hail and torrential rain.

A low pressure system deepening off the coastline has prompted a severe weather alert for damaging winds in Sydney, Newcastle, Toronto, Wyong, The Entrance and Woy Woy.

Bureau of Meteorology duty forecaster Helen Kirkup urged people to secure belongings on their properties and be vigilant ahead of the damaging winds.

A southerly buster is set to build on Thursday, averaging 60 to 70km/h with peak gusts in excess of 90km/h before conditions ease on Friday as the system moves offshore.

"When the gusts hit the coastline, there is the risk of things flying off balconies," Ms Kirkup said.

There is also a risk of large hail, heavy rain and damaging winds for the Mid North Coast and Northern Rivers region, she said.

Marine wind warnings are in place for gale-force winds off Sydney, the Hunter, Illawarra down to Eden and Bateman's Bay with hazardous surf conditions expected.

The State Emergency Services (SES) has urged people to move vehicles under cover away from trees and secure loose items around homes and backyards and keep at least 8m

away from fallen power lines.

Conditions are expected to remain cool and cloudy throughout the day with a top of 20 before the sun brings warmer conditions in time for the weekend.

Originally published as 'Things will go flying': Gale-force winds head for coast