After a brief reprieve from the rain at the weekend, widespread rain returned on Wednesday.
"This is not good news for our already flood-affected areas," the Bureau of Meteorology's Dean Narramore said.
Rain and thunderstorms developing over central Australia will begin to move toward eastern states, including NSW and Victoria, where major flooding is already occurring.
"As we move into Wednesday, we'll see that system move further eastwards, bringing widespread rain and storms," he said on Tuesday.
Thursday threatens "quite severe" thunderstorms through eastern Queensland and NSW, set to continue on Friday.
A low pressure system will weaken but be followed by more rain at the weekend.
The downpours could cause renewed flooding along rivers in parts of the northwest, central west and southwest inland catchments, which are already inundated from months of persistent rain.
Residents in East Moama, near the Victorian border, have been told to evacuate by 1pm on Wednesday as the Murray River continues to rise.
Already 300 people in and near Moama have already been told to evacuate this week, 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 SES has distributed tens of thousands of sandbags and set up evacuation centres in the area.
Deputy State Emergency Services Commissioner Ken Murphy said weary volunteers have been dealing with floods since February and everyone needs to prepare for a long, wet summer.
"What we need to be mindful of is this is a very, very long event and broadly across all of NSW, these floods will continue for a number of months," he told ABC TV on Wednesday.
Emergency Services Minister Steph Cooke said the wet conditions have tested the resolve of flood-weary communities.
"This is what living through a third consecutive La Nina event looks like," she said.
"Our grounds are saturated and we are continuing to see rain and more rain".
Major flood warnings remain in place for eight rivers around the state, mostly inland rivers including the Murrumbidgee and Murray Rivers.
Closer to the coast in the NSW Hunter region, the bureau warns of a renewed flood threat on some rivers as rain continues, with the flood-prone Wollombi Brook likely to flood later in the week.
The bureau is also keeping a close eye on the Lower Hunter River and Colo River north west of Sydney.
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