Emergency crews brace for more wild weather on Australia's eastern seaboard on Tuesday after a storm cell has caused flooding, damage to homes and a mini-tornado in New South Wales.

Authorities in New South Wales (NSW) have responded to 600 calls for help, rescuing 34 affected by floods as heavy rain and damaging winds have lashed parts of NSW over the past 24 hours.

Local authorities have ordered an evacuation of residents in Kiama over fears a local dam wall will fail due to the severe weather in the area, while 50 homes in the same region have become isolated due to flood waters, local media reported on Monday.

The dam is currently being decommissioned, however authorities are concerned the rapid flow of water will scour the remaining dam wall to a point where its integrity will be compromised.

"It's a precaution at the moment but there is a real threat at this stage that we may have some issues further downstream," State Emergency Services (SES) incident controller Ashley Sullivan told local media.

In Sydney, hailstones as large as 20 cent coins were reported in some suburbs as heavy rain and hail swept the city late on Monday.

The severe weather caused flash flooding throughout the city's Central Business District (CBD) as well as the inner west and eastern suburbs, before moving south to the Kiama area and out to sea.

Meanwhile, a mini-tornado has hit inland New South Wales tearing the roof off buildings and bringing down trees and power- lines in the country town of Dubbo.

"What we saw develop was basically a mini tornado and we saw large sheets of iron and building material and branches of trees get sucked back into the air up into the storm cell," Fire and Rescue NSW Dubbo station manager Mick Medlin told Australia's national broadcaster.