Nine emergency aircraft are on standby, along with SES Unimog flood rescue vehicles, and 880 ADF personnel have been requested by the state government to help high-risk communities in the state's far west and south over the coming days.
ADF troops are helping threatened communities with sandbagging and doorknocking, working from a base camp set up at Deniliquin near the Victorian border.
"Almost every river system west of the Dividing Range is in flood to some extent at present," Flood Recovery Minister Steph Cooke said on Wednesday.
"It doesn't take much rain, to cause those rivers to rise and rise very quickly. Our SES volunteers have been at this effort for a very long time ... in some cases, well over 12 months.
"They are weary, understandably and so we're very grateful to the federal government and the ADF for their support during this time."
More thunderstorms are heading for NSW on Wednesday, as a new rain-bearing system brings the threat of short, sharp intense bursts of rain to saturated catchments, Jane Golding from the Bureau of Meteorology said.
"There's a very real risk of flash-flooding across western NSW today from thunderstorms," she said.
The storms will also bring a risk of destructive gusts with winds of up to 125 kilometres and hour downed trees, hail, and generally increased showers.
The highest rainfall totals - close to 100 millimetres in some areas - are expected on the north west slopes and plains.
It comes as six major flood warnings are already in place, for water that has already fallen and is yet to dissipate.
Major flooding is expected on the Murray River at Moama and Echuca in Victoria, but the water level is not expected to peak until Thursday or Friday.
Already 300 people in and near Moama have been told to evacuate and thousands are on stand-by with the area predicted to see flood heights eclipse the area's second-worst on record in 1993.
The Murrumbidgee River is also rising at Narrandera, with major flooding expected at Hay in the coming days.
Moderate flooding continues at Forbes in the central west with the Lachlan River likely to peak at Condobolin on Friday.
Flooding also continues at Warren, where the Macquarie River has been at major levels for weeks.
Major warnings are also in place for Bourke, Tilpa and Mungindi, as several flood peaks flow through tributaries to the Barwon and Darling Rivers.
The SES is already ferrying emergency supplies to communities that have been cut off by floodwaters and that service is likely to be required for weeks.
Some 170 rescues had been carried out in recent days, solely from people driving into floodwaters and SES Commissioner Carlene York urged people to heed the advice evacuate when advised to do so.
"I remind people that the (flood) waters are contaminated, so it is not healthy at all to go into the waters, or to walk through the waters," Ms York said.
"If we are giving you a warning to evacuate, please do so whilst it's safe to do so and don't wait until you think that it's now a risk."
