Heaviest deluge in 80 years force NSW evacuations

The floods in NSW come off the back of flash flooding on the Sunshine Coast at the start of the week. Photo / Courier Mail

Evacuations are underway in one Australian town and emergency crews are standing by in three others as large parts of NSW continue to be deluged with the heaviest rains in more than 80 years.

Seventy residents have been forced from homes on 19 properties at Cowra in the state's central west, with rising flood waters threatening to leave them stranded, the State Emergency Service says.

Evacuation warnings are current for Cooma in the Snowy Mountains, with up to 300 people at risk, and parts of the southern tablelands township of Goulburn, where the Wollondilly Creek is threatening to burst its banks.

Orders are also in place at Captains Flat south of Queanbeyan, while crews are keeping an eye on the swollen Murrumbidgee River at Cootamundra in the Riverina, an SES spokesman says.

Sydney's Warragamba Dam is also on the verge of overflowing for the first time in more than a decade.

Sydney Catchment Authority acting chief executive Sarah Dinning says preparations are being made to release excess water, with floodgates to be tested on Thursday morning.

"Due to the variable weather conditions, we have staff available around the clock and the test will occur as soon as the dam reaches one metre below full storage,'' she said.

"Once Warragamba Dam is 80mm above its full storage level the drum gate opens automatically.''

Heavy falls are expected to continue until at least the weekend, throwing transport into chaos across NSW and stretching emergency crews.

A number of highways have been cut off with previously drought-stricken areas swamped with rain.

The Illawarra Highway south of Sydney was closed on yesterday while the Barrier Highway in the state's far west was also cut off, stranding travellers between Broken Hill and the South Australian border as the mining town received its heaviest downpour in 12 years.

Longer standing rain records were also broken, with Coolamon in the Riverina bucketed with 123mm in 24 hours, marking the wettest day since 1925. Nearby Grong Grong recorded 111mm, the wettest day since 1928.

SES crews remain on standby to evacuate residents at Cooma if flash flooding occurs.

''(Cooma Creek) might flood six times or it might not flood at all,'' SES spokesman Phil Campbell said.

"But that little creek will rise very quickly and flood those properties within a matter of hours if there is heavy rainfall.''

In the 24 hours to yesterday morning, the SES received 400 calls for help, mainly in the storm-hit areas of Cooma and Cootamundra.

Weather bureau duty forecaster Jake Phillips said an unusually slow-moving trough was bringing heavy rains to the southern half of the state, which would continue until at least Saturday.

"If you drew a line from somewhere near Broken Hill and across to Sydney ... any of those regions are looking like they'll get 100 to 300mm,'' he told AAP.

"It looks like hanging around until the weekend; there's a chance of showers clearing on Saturday or maybe Sunday when this trough looks like pushing out to the Tasman.''

- AAP