'Dangerous' storms to hit NSW towns

Flood-prone NSW communities are being warned to brace for a "dangerous" 72 hours, as widespread rainfall and thunderstorms are set to wash over the state.

The State Emergency Service has issued warnings that towns could face renewed flooding to levels experienced over the past few weeks.

"This rainfall is combining with already saturated catchments and full river systems which will lead to more flooding across many parts of the state," NSW SES Commissioner Carlene York said in a statement on Friday.

People are being urged to watch out for landslips and falling trees, as authorities warn of flash flooding from short bursts of rain in some areas.

"The situation can evolve quickly so it is important people remain aware of their surroundings, monitor conditions and warnings and if asked to evacuate act quickly," she said.

More flood-affected NSW residents can now access the federal government's one-off disaster recovery payment after the scheme was extended to an additional five local government areas on Friday.

People living in Cootamundra-Gundagai, Forbes, Liverpool Plains, Snowy Valleys and Upper Lachlan will be able to apply for $1000 per adult and $400 per child.

The Commonwealth has also extended financial support to workers and sole traders in 11 NSW local government areas who lost their income due to the floods.

People in the Bega Valley, Junee, Muswellbrook, Nambucca Valley, Shoalhaven, Snowy Monaro, Snowy Valleys, Temora, Walcha and Yass Valley and the Unincorporated Area of NSW will be able to access a fortnightly payment for up to 13 weeks.

"We don't want people to put themselves at risk trying to get to work and we don't want to leave businesses directly affected by this disaster out-of-pocket," federal Emergency Management Minister Murray Watt said.

The SES has conducted 400 damage assessments in the central west NSW town of Forbes, with significant damage to a number of homes and businesses.

Downstream, the town of Condobolin is expected to see flood levels reach similar to the 1952 record heights.

Urgent supply drops have continued in the northern NSW towns of Walgett, Collarenebri, and Lightning Ridge, with the SES helping with medical evacuations.

The agency issued its one-millionth sandbag across the state on Friday.

"It is important people who have sandbags to keep them in place, given we're expecting renewed river rises and additional flooding to many towns which have already seen flooding," Ms York said.

Meanwhile, Sydney is becoming more vulnerable to flash flooding as new research reveals an uptick in heavy bursts of rain over the past two decades.

Researchers from the ARC Centre of Excellence for Climate Extremes found rain bursts, which occur over a period of 10 minutes, have intensified in Sydney by 40 per cent over the past 20 years.

The rapid rain bursts bring a large amount of water falling rapidly over a small region, increasing the likelihood and severity of flash-flooding.

The discovery means city planners must re-evaluate how they design their infrastructure in order to withstand the wetter conditions, UNSW climate researcher Jason Evans said.

"The potential for very localised flash flooding is much higher now than it used to be," Professor Evans said.