A humid air mass has brought a severe storm risk to numerous parts of the country, worsening the flood risk on already saturated catchments across the east coast.
Bureau of Meteorology hazard preparedness and response manager Steve Bernasconi says severe thunderstorms and widespread rain are expected on Sunday.
"All of this adds to the existing, extended flooding that's happening in inland NSW," he said.
Heavy falls are forecast for parts of NSW and Victoria on Sunday, while a moist, humid air mass tracks north east, bringing isolated storm cells to parts of Queensland and the Northern Territory.
Conditions are expected to ease around Tuesday with cooler, drier and more settled conditions.
The NSW SES has established a flood rescue operation centre at Goulburn and is positioning personnel and vehicles in areas of concern, including Albury, Young, Yass, Wagga Wagga and other towns along the Murray River system.
A severe thunderstorm warning has been issued in central NSW, for the South West Slopes, Central West Slopes, Central and Southern Tablelands, Riverina and Snowy Mountains regions, with heavy falls that could lead to life-threatening flash floods forecast for Sunday evening.
The SES has 75 warnings in place including five emergency warnings to evacuate.
Commissioner Carlene York urged people to follow the SES warnings and evacuation orders.
"Some people are still not listening, but they make, regretfully, their decisions based on what they've seen in the past," Ms York said.
"But we've seen time and time again the water is rising further than perhaps what it has for many years and that's because of the saturation of the land."
The volunteer workforce is fatigued and personnel have been requested from other states, she says.
NSW Emergency Services Minister Steph Cooke says communities around the state are fatigued too, including in Gunnedah, which she visited on Saturday.
"That community has experienced eight floods in the last 12 months and it's taking its toll on people," she said.
"There's no silver bullet to what we're experiencing at present and will likely continue to experience for a little while yet."
The SES is continuing resupply operations delivering food and medicine to flooded communities, covering more than 40,000 square kilometres, roughly the size of Switzerland, Ms Cooke says.
Ms York says some communities will remain isolated for weeks, if not months.
In Victoria's northeast, intense showers, driven by storms is expected to bring flash flooding, with falls of up to 150mm in 24 hours possible in alpine areas.
A severe weather warning has been issued for possible flash flooding in East Gippsland, South Gippsland, Northern Country, North Central and the Central West on Sunday afternoon.
Particular areas of concern include Seymour, Wodonga, Wangaratta, Corryong, Bright, Mansfield, Falls Creek, Mt Hotham and Mt Buller.
Major and moderate flooding is occurring in northern Victoria and the rain, combined with an already saturated catchment area and dam releases, is expected to prompt further and fast rises across parts of the Murray and Edward rivers.
In the Northern Territory, severe storms are developing in Carpentaria and parts of the Arnhem and Barkly districts on Sunday afternoon, as a humid mass of air moves along a surface trough.
Heavy falls with possible flash flooding is predicted in Borroloola, Mataranka, Ngukurr, Larrimah, Beswick and Wollogorang.
In Queensland, severe storms are forecast on Sunday, and are expected over Mount Isa, Cloncurry, Dajarra, Duchess, Selwyn and Mckinlay in the state's far north.
The intense storms hitting the eastern states passed through South Australia on Saturday, with more than 423,000 lightning strikes and heavy winds felling trees and damaging electricity infrastructure.
SA Power Networks estimated on Sunday 100,000 customers had their supply impacted and has called in electrical crews from interstate to help with ongoing restoration work.
