Severe storm warnings issued for New South Wales

Severe thunderstorms are sweeping across Sydney from the north west, with flash flooding and hail large enough to "cut and bruise".

The NSW State Emergency Service (SES) was bombarded with calls for assistance between 1.30pm and 2pm (local time) on Saturday as torrential rain lashed a wide area.

At Hazelbrook in the the Blue Mountains, west of Sydney, two adults and two children were rescued from their flooded home, SES spokesman Dave Owens told AAP.

"They are quite severe weather systems," he said.

"They are definitely giving us an immediate effect."

"I'm in Hazelbrook at it is horrendous," Lisa Maxwell wrote on the SES' Facebook page.

"The hail was large enough to cut and bruise. Our house is flooded, and we're high, so I imagine it's awful for those in lower parts.

The SES warned at 2.35pm that severe storms were battering Sydney.

"Put your car under cover and bring children and pets indoors. Stay away from flash flooding," it advised.

The Bureau of Meteorology (BoM) sent out an alert that at 2pm warning of severe thunderstorms detected on weather radar near Colo, Upper Colo and Wilberforce north-west of Sydney.

The thunderstorms are heading, the BoM said.

The wild weather was forecast to hit St Albans, the Yengo National Park northwest of St Albans, Maroota and Wisemans Ferry by 2.30pm.

Thunderstorms will also move towards Calga, Upper Mangrove, Mangrove Mountain and Mangrove Creek Dam by 3pm, the bureau predicated.

A warning for issued to prepare for large hailstones, very heavy rainfall and flash flooding.

-AAP