At least 50 people in Thargomindah in Queensland's southwest were evacuated on Monday morning as the flooding threat spread in the sodden state.
Rainfall has finally eased after days of downpours in central and southwest Queensland but floodwaters may plague the region for weeks, inflicting widespread devastation.
A temporary levee bank was built at Thargomindah as many took shelter after the floodwaters rose.
"They're expecting seven metres of floodwaters in Thargo over the next few days," AgForce general president Shane McCarthy told AAP.
"There's unprecedented levels of flooding out there."
Repair work has reportedly already begun on the levee bank constructed around Thargomindah as locals try and keep the floodwaters at bay.
There are fears hundreds of thousands of livestock may be lost in floodwaters not seen in the region for more than 50 years.
"A lot of those communities have just been inundated with water that they've never seen before - 1974 was generally the benchmark, we've gone past that and a bit," Mr McCarthy said.
Many in the state's central and southwest have already been evacuated from floodwaters in recent days, with outback town Adavale's entire population of about 30 relocated by helicopter.
Others hit hard in the southwest are Windorah and Quilpie with major flood warnings current for a string of rivers across the region.
"This floodwater is very slow to subside, so we are likely to see these major flood warnings continue for quite some time," the Bureau of Meteorology's Jonathan How said.
Floodwaters would need to subside before gauging just how widespread the devastation is on livestock and properties, Mr McCarthy said.
"The only really way to get around everywhere is by light plane or helicopter," he said.
"Fodder drops have commenced and at the moment they will be only focusing on the stock that are still alive, and trying to ... move them to higher ground.
"They won't even be looking at the stock losses at the moment."
The federal and state governments have activated personal hardship assistance along with concessional loans and freight subsidies to help primary producers in a string of western Queensland communities.
In NSW, there are concerns for an elderly man who went missing in flooding that has isolated communities after widespread rainfall.
Communities along the Paroo and Warrego Rivers were warned they may be cut off for up to six weeks.
"We need you now to start preparing for that isolation," NSW Emergency Services Deputy Commissioner Deb Platz said.
Authorities are still searching for an elderly man who tried to drive across a flooded causeway 100km west of Taree late on Saturday night.
His SUV, which was towing a caravan, was swept into the floodwaters and neither the driver or car have been found.
Moderate flood warnings for the Namoi, Narran and Warrego Rivers were current on Monday, the bureau said.
A severe weather warning has also been issued for coastal areas with wind gusts exceeding 90km/h recorded south of Sydney on Monday morning.
Winds are set to gradually ease but hazardous coastal conditions will continue into Tuesday, with the risk of damaging surf, Mr How said.
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