Federal Emergency Management Minister Murray Watt says the clean-up bill will be staggering, with $3 billion allocated for disaster payments and repairs alone.
He was speaking as central western town of Forbes becomes the focus of emergency services efforts on Thursday, with days of flooding predicted.
Hundreds of homes and businesses are underwater - some for the second time in as many weeks.
"The overall cost of the current flood disaster, I can guarantee you, is going to be in the billions of dollars," Mr Watt told reporters in Brisbane.
"With each week these the floods go on and with each road damaged, the damage bill is going to go up."
Mr Watt said dozens, possibly hundreds of homes in the decimated town of Eugowra, which was hit with catastrophic flash flooding on Monday, have been deemed "uninhabitable".
"You've literally got homes that have been washed away ... they've been dislodged and moved in some cases tens and hundreds of metres down the street."
The Lachlan River is flowing through the centre of Forbes, inching closer to a predicted peak of 10.8 metres on Thursday, a similar level to the historic inundation of June 1952.
Around 1000 people have evacuated hundreds of homes.
About 85 kilometres west of Forbes, the entire town of Condobolin has been cut off due to flood levels not seen before, Lachlan Shire mayor John Medcalf said.
"We've got a situation now which is just an absolute natural disaster," he told AAP on Thursday.
"At the moment, the flood is at one of its highest peaks it's been for many, many years."
With the airport being the town's last port of contact, the community is relying on emergency services to airdrop supplies after fuel levels and basic products such as milk ran low, Mr Medcalf said.
Agricultural areas have been devastated by the inundation with some farmers losing entire crops, ahead of harvest season.
Meanwhile, the body of 60-year-old Dianne Smith was recovered from floodwaters in Eugowra, after the town was devastated by a deluge of water that brought roof-high flash-flooding in the early hours of Monday.
Ljubisa "Les" Vugec, 85, last seen at his Eugowra home around the same time, is still missing.
Authorities conducted 284 property assessments at Eugowra on Wednesday and determined the vast majority of those buildings were damaged.
Residents described two sudden and intense surges of water, which washed away houses, knocked over structures and left destruction like a "war zone".
The SES performed 12 flood rescues and handled 265 requests for help in the 24 hours until Thursday morning.
Across the state, authorities have issued 113 warnings and 23 evacuation orders, including for the central-western towns of Forbes and Gooloogong, Gunnedah in the northeast and the southern border town of Moama.
Meanwhile, 18 flood rescue specialists have arrived from the Singapore Civil Defence Force to relieve exhausted workers who have been dealing with 65 consecutive days of flooding.
SES Chief Superintendent Ashley Sullivan says some rivers will remain in flood throughout the summer months to come.
"So this interstate and international support is critical to making sure we can last the long haul," he told ABC TV.
The international flood rescue operators will be deployed to Parkes and Wagga Wagga.
Along the Lachlan, major flooding is also occurring at Nanami, Cottons Weir, Jemalong and Hillston.
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