Hail and heavy rain pelted parts of Sydney on Monday night, with hailstones as large as 20 cent coins reported in some suburbs.

The extreme weather caused flash flooding throughout the CBD and several surrounding suburbs, causing major traffic delays, according to the state's Transport Management Centre.

The Bureau of Meteorology said the Illawarra and South Coast would bear the brunt of the wet and windy weather throughout Tuesday, after a small low pressure centre developed off the northern Illawarra coast overnight.

"This pattern is generating vigorous onshore winds along the southern half of the coast, and bringing periods of heavy rainfall to the region," the bureau said in a statement.

"The low is expected to reach peak intensity later today, before gradually weakening during Wednesday."

A severe weather warning for damaging winds and heavy rainfall has been issued for the Illawarra and South Coast forecast districts on Tuesday. Bellambi, north of Wollongong, was drenched by 177 millimetres of rain between 9am on Monday and 6am on Tuesday, while Nowra received 160 millimetres of rain and Albion Park 140 millimetres in the same period.

In Sydney, rainfall recorded at Observatory Hill jumped from 17 millimetres to 54.2 millimetres in just 41 minutes between 8.49pm and 9.30pm on Monday, according to the Bureau of Meteorology. Little rain fell after that time.

Old mate has pulled over but not sure if it's very safe atm. https://t.co/1GEOL3KzAP— Mark Di Stefano (@MarkDiStef) August 24, 2015

By 6am on Tuesday, NSW State Emergency Service workers had received nearly 600 calls for assistance due to the torrential rain and hail throughout Sydney and the Illawarra.

Sydney #hailstorm right now #hail #hailstones @cityofsydney @sydney_sider pic.twitter.com/6JIvjP02SZ— Migration Alliance (@lianaallan) August 24, 2015

Flood rescues accounted for 34 of those call-outs. Almost all were due to people driving into flood water.

"We've been pulling people out of cars all night and we're concerned people are not heeding the message to avoid flood waters," said SES spokesman Phil Campbell.

"It can be difficult to judge in the dark, but in some cases people have deliberately entered flooded roads."

Starting to feel a little nervous here in Paddington #sydneystorm pic.twitter.com/mtwI8F8bsI— Drew McKillican (@mckdrew) August 24, 2015

Train services on the South Coast Line between Coalcliff and Thirroul were suspended at the height of the storm on Monday night.

At 6.30am on Tuesday, Sydney Trains warned passengers on the South Coast Line to allow additional travel time after early track equipment repairs at Minnamurra, near Kiama.

"Buses may supplement some services so passengers should also listen to station announcements and check indicator boards," Sydney Trains said in a statement.

A tornado in Dubbo damaged housing, scattering debris, up-rooting trees and scaring residents late on Monday afternoon.

Dubbo State Emergency Services regional controller David Monk said the SES was responding to 10 calls for assistance as of 6.30pm.

"It cut a path about 200 metres wide with another 100 metres either side showing debris," Mr Monk said.

A top of 18 degrees is forecast in the city and in Sydney's west on Tuesday with a high chance of showers, possibly increasing to periods of rain. There is also a chance of a thunderstorm near the coast, the bureau said, while winds could gust up to 50 km/h in the late afternoon or evening.

Rain is forecast again in Sydney on Wednesday, easing to possible showers on Thursday, and cloudy conditions on Friday.

The Transport Management Centre said the Audley Weir was closed in both directions in the Royal National Park on Tuesday due to heavy rain.

"The weir will be regularly assessed and when deemed safe will reopen. There is no forecast available at this stage as to when the weir will reopen," a spokesman said.