Wagga Wagga in southern New South Wales is expected to be inundated today by the highest floodwaters since 1853.
Eight-thousand residents are evacuating the central business district and Flowerdale areas as the Murrumbidgee River threatens to breach the Riverina city's levee banks.
The Bureau of Meteorology expects the river to peak at 10.9 metres by about 9am (11am NZT) this morning, reaching a level last recorded in Wagga Wagga in July 1853.
That level would only be marginally below the all-time record of 10.97 metres reached in October 1844, data from Wagga Wagga City Council shows.
The latest evacuation in Wagga Wagga amounts to the biggest emergency evacuation since heavy rains began falling last week.
State Emergency Service region controller James McTavish told residents to leave by dawn.
"Don't delay. This is a very serious issue and if you live in any of the areas that are subject to an evacuation you must be clea ...by 6am," he told reporters last night.
Victoria hit hard
In Victoria, emergency crews are working to protect homes in the northern town of Nathalia before floodwaters peak on Thursday.
Crews are trying to secure the levee in the town, which is located north of Shepparton and yet to see the worst of the once-in-a-century floods.
They are trying to protect 176 homes before floodwaters are expected to peak on Thursday at between 3.2m and 3.35m.
Up to 300,000 sandbags are being used across Victoria's flooded northeast and several residents are likely to remain isolated for days.
The State Emergency Service (SES) says roads in Numurkah will remain under water for several days after flooding topped 1974 and 1993 levels at the weekend.
Thirty-eight residents at the Numurkah Pioneers Memorial Lodge aged care facility are isolated but have supplies to last several days, an SES spokeswoman said.
The isolated residents would likely be evacuated if supplies run out.
Death and despair in Queensland
Meanwhile, with one life already lost, residents in Queensland's sodden southeast are praying there will not be a repeat of last year's devastating floods.
A man died at Glenwood, between Gympie and Maryborough, after the car he was in was washed off the road during the deluge yesterday morning.
The Department of Community Safety said bystanders spotted another person nearby. "They've said there was a good Samaritan there on the scene when the four-wheel-drive went over the causeway, but we've been unable to confirm those reports," a department spokeswoman said.
Authorities have called off the search for a third person after it transpired that another car had been unoccupied when it was swept away in the deluge.
Gympie Regional Council Mayor Ron Dyne said that "extreme rainfall" overnight flooded creeks and some homes.
But he says the flooding will not be as bad as that which devastated most of southeast Queensland in January last year, so long as the Mary River does not overflow.
Earlier, a father and son stranded on top of their car were plucked to safety from racing floodwaters at Tiaro. The pair, aged 26 and 65, waited more than three hours before two crews in flood boats were able to retrieve them.
Nine more local government areas have been declared disaster zones.
The Riverina region, in the state's southwest, has become the focus for emergency services, with the Murrumbidgee River bursting its banks.
Downstream at Gundagai, about 60 people have been evacuated with 30 homes, businesses and the local racecourse flooded.