Aerial supply missions are being conducted in flood-stricken western NSW as isolated communities brace for renewed inundation.
SES Assistant Commissioner Nicole Hogan says the situation is complex and evolving for towns like Collarenebri, Walgett, Lightning Ridge, Goodooga, Brewarrina and parts of Bourke.
"The size of the area is about 40,000 square kilometres - more than half the size of Tasmania," she said on Saturday.
"Roads have intermittently been cut for several weeks, with re-supply missions needing to take place by airplane to regional depots before being loaded onto helicopters and finally transported by vehicles and boats."
With further flooding expected, regular missions are being flown to get essential food, medicines and mail to around 5000 stranded residents.
"As the flood water recedes we hope access will improve, however if people are in need of re-supply they should get in touch with the NSW SES," she said.
Volunteers have also been doorknocking, sandbagging and conducting flood rescues in central western Condobolin as flooding persists on the Lachlan River.
Ms Hogan said conditions were likely to decline over the coming 48 hours, with heavy rain and severe storms to trigger further inundation and dangerous flash flooding.
She said the SES had performed five rescues and received 145 requests for help in the 24 hours to Saturday morning.
Major flood warnings are active across NSW for the Barwon, Darling, Macquarie, Lachlan, Murrumbidgee, Murray, Bogan, Namoi, Narran and Culgoa rivers.
More than 90 hazard alerts are current, with evacuation orders in place for Condobolin, Collarenebri in the northwest and the Alice Edwards Village at far western Bourke.
Meanwhile, an additional four local government areas (LGAs) have been added to jointly funded assistance from the federal and NSW governments.
Emergency Management Minister Murray Watt said on Saturday the LGAs of Bathurst and Oberon - which have been affected by a major gas pipeline leak - along with Shellharbour and Uralla have been included in a natural disaster declaration (NDD).
"With such a massive task ahead of them, these councils need the Australian and NSW governments to work collaboratively and quickly so they know they've got the financial backing to get on with the job," he said.
NSW Emergency Services Minister Steph Cooke said 70 LGAs are now covered by a NDD.
Victoria's northeast is also bracing for another emergency with widespread showers and storms likely to cause moderate to major flooding particularly about central and eastern ranges.
Up to 90 millimetres of rain is forecast for some areas.
Heavy rain and large hailstones are also a possibility for the rest of the state.
More than 50 emergency alerts are in place, with a major flood warning still active for the Murray River.
It's feared water could rise to about the same level as the devastating 1975 floods in low-lying areas along the river such as Mildura, Robinvale, Boundary Bend and Wakool Junction.
Cooler air will start to extend across Victoria into next week, leading to colder temperatures in Melbourne and snow returning to some of the state's alpine peaks.
Forecasters expect above average rainfall in Victoria to persist for another six to eight weeks.
A watch and act alert has also been issued for Upper River Murray in South Australia.
Flows beyond the border with NSW and Victoria are at 92.5 gigalitres a day and expected to reach 165GL per day by December.
Authorities say there's a risk the volume of water will reach 220GL a day, which would pose a threat to public safety.