A severe weather warning has been issued to all towns and communities stretching almost 1,000 kilometres of the NSW coast, with heavy rain, possible renewed flooding, land slips and strong winds.
The Bureau of Meteorology has numerous flood warnings along the coast and has also forecast possible severe thunderstorms, hazardous surf, damaging wind and hail.
It also forecasts possible severe thunderstorms later this week in southeast Queensland, with soaked catchments ripe for flash flooding.
Floods have claimed six lives in NSW, including four in Lismore, while Queensland's death toll for the disaster has risen to 12 after a man in his 60s was found dead in a car submerged in floodwaters.
His body was found late on Sunday night in Wondai, in the South Burnett Region, the Queensland Fire and Emergency Service confirmed on Monday.
A woman, also in her 60s, who was a passenger in the car, managed to get out and was found clinging to a tree.
An elderly man remains missing after falling from a boat into the Brisbane River near Breakfast Creek nine days ago.
Broad areas of NSW east coast were facing "a very serious situation", the BOM's Dean Narramore said on Monday.
The warnings extend to "every community and town in between and extending well inland towards the Blue Mountains, Central Tablelands, Southern Highlands and possibly Canberra", he said.
State Emergency Commissioner Carlene York warned the Hawkesbury River was of prime concern.
"We're looking at flooding similar to last week on some parts the river. In other areas it could be worse than what we saw last weekend - similar to what we saw in March last year," she said.
"If it is not safe to go home please do not go home.
"Stay where it is safe and make decisions that are safe for you and your families."
Floodwater from the swollen Hawkesbury river is seen at Windsor, north west of Sydney. A developing low pressure system is expected to intensify overnight impacting Sydney with heavy rain that could cause flash flooding and potentially hazardous condition. Source: AAP / DAN HIMBRECHTS/AAPIMAGE
The heavy rain is forecast to increase across the Hunter on Monday and move towards Sydney, Hawkesbury Nepean and spread to the Illawarra and Shoalhaven.
Heavy rain will ramp up in the early hours of Tuesday and continue most of the day, Mr Narramore said.
"We are likely to see major flooding on numerous rivers, much of the flooded catchments that does include the Hawkesbury and Nepean."
A road weather alert has been issued for metro Sydney through to the Central Coast and Illawarra, with a warning heavy rain and strong winds could down trees and power lines, cut roads and cause land slips.
The warnings come as communities in the Northern Rivers continue to mop up from last week's unprecedented flooding.
Australian Defence Force Personel assist with the clean up after the floods in the Central Business District of Lismore, NSW. Source: AAP / JASON O’BRIEN/AAPIMAGE
"We have resources from the ADF that they will be 2,000 on the ground by the end of the day into tomorrow."
Premier Dominic Perrottet has apologised to thousands of people in the Northern Rivers whose homes and businesses were destroyed when towns like Lismore, Ballina and Mullumbimby were inundated last week.
Many had to be rescued by fellow citizens and have since struggled to access basics such as food, water, power, fuel, phones, ATMs, the internet and medical supplies.
"I'm very sorry ... but we're doing everything that we can," Mr Perrottet told the Nine Network from Lismore.
"Some of those areas will look back at this and say it could have been done better but ... we're doing absolutely everything we can to provide care and support for those who need it to get those essential supplies in," he said.
Around 2,000 of the 3,500 homes assessed so far in the Northern Rivers are no longer habitable, a number expected to rise, increasing the need for short-term and medium-to-long-term accommodation.
The SES received 1,290 calls for help in the last 24 hours and carried out 25 rescues in the same period.
Job protection has been announced for flood emergency volunteers, to protect people from being sacked or demoted as they help with the clean-up.
Queensland floods to cost state $2.5 billion
Queensland Treasurer Cameron Dick said the floods, which have been going for two weeks but peaked last weekend, will cost the state at least $2.5 billion.
Treasury estimates the cost of fixing damaged public infrastructure like roads, bridges, railways and ports will be at least $500 million, while the insurance claims on damaged homes and businesses are likely to top $936 million.
Mr Dick said the floods are also expected to wipe 0.25 per cent, or $1 billion, in economic growth from the current quarter.
"Although the weather event was pretty short, it was only about three days in duration, the cost of this weather event will be very significant," he told reporters on Monday.
Mr Dick said the $2.5 billion estimate is only preliminary and he expects the damage bill will increase, particularly as people cleaning up their homes and businesses lodge insurance claims later and councils make more detailed assessments.
He said the 2022 floods won't cost as much as the 2011 floods or ex-tropical Cyclone Debbie in 2017, Oswald in 2013 or Cyclone Yasi in 2011.
"But for people who are throwing out treasured possessions or are hosing out mud from their homes in stinking heat, it doesn't really matter which flood costs more," Mr Dick said.
The treasurer has ruled out any new taxes or a levy, like the one brought in after the 2011 floods, to pay the bill, and he expects the state to return to surplus in 2024/25 as planned.
"We're holding tight to that promise, notwithstanding the natural disasters, but we never know what's around the corner when it comes to natural disasters," Mr Dick said.
More than 1,350 Defence Force personnel are helping with the recovery effort, focusing on the towns of Gympie, Gatton, Esk and Grantham as well as suburban Brisbane and Logan.
The total estimated cost of insurance claims in NSW and Queensland rose to $1.3 billion, the Insurance Council of Australia said.
Some 86,703 claims have been lodged, 28 per cent from NSW.
Telstra said it had restored about 80 per cent of mobile coverage in northern NSW and 75 per cent of landline connections, however continued flood and road damage continued to hamper their technicians.
