A humid air mass has brought a series of severe storm warnings to numerous parts of the country, worsening the flood risk on already saturated catchments across eastern Australia.
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 large tracts of NSW and Victoria on Sunday, while a moist, humid air mass moves north east, bringing isolated storm cells to parts of Queensland and the Northern Territory.
Thunderstorms are expected to hit a large warning area in NSW on Sunday evening from Wanaaring in the far northwest through to Cobar, Orange, Bourke and Parkes in central NSW, through to Wollongong on the south coast.
Storms will to continue into next week in regional NSW, bringing fresh concerns to already-inundated towns, the NSW State Emergency Service warned on Sunday.
"We understand this is the last thing you want to hear, with flooding affecting multiple communities in inland NSW," NSW SES Assistant Commissioner Nicole Hogan said.
The Lachlan River was already rising at Forbes in the state's central west, after experiencing a historic flood one week ago, while a major peak was also occurring at Condobolin, she said.
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, Ms York said.
Flooding continues across other parts of western and southern NSW, in towns including Collarenebri, Walgett, Bourke, Hay, and Albury.
A cold front is expected to sweep across NSW on Sunday continuing on Monday, bringing storms and adverse weather.
Snow is expected in the alps and Central Tablelands.
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."
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 floods 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.