Sydney has been by hit thunderstorms and damaging winds of more than 100km/h, closing runways at the airport and causing significant delays, with the weather system now expected to move to the state’s saturated north.

The Bureau of Meteorology initially issued a severe thunderstorm warning for large parts of Sydney on Monday, including Bondi Beach, but that was downgraded by 11.30am.

About 6pm on Monday, a severe thunderstorm warning remained in place for the Northern Tablelands and parts of the Northern Rivers, Mid North Coast and North West Slopes and Plains.

Residents were warned to expect damaging winds, large hailstones and heavy rainfall that may lead to flash flooding in Grafton, Coffs Harbour, Armidale, Tenterfield, Sawtell and Dorrigo.

In Sydney, the bureau had warned of damaging winds of about 100km/h in areas where the thunderstorms hit. Fort Denison, in Sydney Harbour, recorded a 94 km/h wind gust, while Kurnell recorded a wind gust of 89 km/h, and Sydney Airport recorded one of 115km/h.

The severe weather forced the closure of all runways at the airport for about 15 minutes, causing delays for travellers.

The State Emergency Service advised people to move cars away from trees and secure loose items around the house.

Weatherzone meteorologist Felix Levesque said the storms were part of a broader system moving across the state, driven by moist, warm air combined with a cooler trough. These two systems were creating instability in the atmosphere, resulting in severe thunderstorms.