Emergency warnings and evacuations orders are in place for areas stretching from Sydney's north-west to the Riverina and central west after wild weather lashed New South Wales.
Dozens of people have been rescued from floodwaters and the SES has fielded hundreds of calls for help.
Emergency Services Minister Steph Cooke said rivers were yet to peak in some regional and rural communities.
"They'll be on edge for the next few days while we await another big system to arrive on Wednesday through to Friday," she said.
Prime Minister Anthony Albanese has urged people in flood-affected areas to heed the warnings.
"I would say to people to listen to the authorities, follow the advice which is there ... this is not a time to take risks," he said.
"My heart goes out to those people who have suffered time and time again, particularly in the recent period."
Rain adds to dam releases
In western parts of NSW, water is steadily rising along the Murrumbidgee River, with recent rain combining with releases from Burrunjuck Dam.
The dam is now releasing 100,000 megalitres of water per day and communities downstream are bracing for a flood peak in the next 24 hours.
That includes Gundagai, where moderate flooding is already happening, with a major level possible on Monday.
Moderate flooding is possible at Wagga Wagga from Tuesday.
Releases from the Wyangala Dam into the Lachlan River have increased from 10,000 to 35,000 megalitres per day.
The dam sits at the top of the waterway which leads to Forbes, where a moderate, "near-major" flood is expected.
The Oura Beach campground, just east of Wagga Wagga, is under an evacuation order and all campers had left the area by Sunday morning.
Oura local Den Fleming said she expected the water there to keep rising.
"The water has come up quickly, in the last couple of days, but I think the actual peak that's coming down the river hasn't got to us just yet," Ms Fleming said.
Sydney's situation deteriorates
In Sydney, the number of emergency warnings is growing.
Throughout the day, numerous notifications were issued for the Hawkesbury, Colo and Macdonald rivers.
On Sunday evening, the SES warned that moderate flooding was expected at North Richmond and Windsor, with the Colo River at Putty Road already impacted.
The Yarramundi Bridge was closed in the early hours of Sunday morning.
Residents of Gronos Point on the Hawkesbury River were issued an order to leave early on Sunday morning, and people at the Riverside Ski Park at Cattai were asked to leave by 6pm.
The northern part of Pitt Town, including Percy's Place Caravan Park and properties along Hall Street, had to be evacuated by 11:45pm on Sunday.
The SES said parts of Lower Portland on the eastern side of the Hawkesbury should evacuate on Sunday afternoon before evacuation routes closed.
Residents in low-lying farmland in Agnes Banks at Richmond on the Hawkesbury River have also been issued with an order to leave due to dangerous flooding.
The SES has warned the Windsor area to expect moderate flooding, with evacuation orders issued for Gronos Farm Road and Manns Road.
Some residents under evacuation orders at Gronos Point said they would not leave their homes.
Resident Peter Worsley said he was getting used to constant floods, but was not expecting water to threaten his home this time.
"We're not panicking, we've seen the worst earlier in July, floods were bad, we were really, really impacted by it," Mr Worsley said.
"But this one's not going to be anywhere near as bad, we're learning to live with them, unfortunately."
The SES has issued Watch and Act warnings for other areas along the Hawkesbury, include parts of Pitt Town, Ebenezer and Upper Macdonald.
Evacuation centres have been set up at North Richmond Community Centre and Castle Hill RSL Club.
Rescues across the state
Falls of 40 to 50 millimetres have been recorded across much of the state already experiencing flooding, and there were some significantly higher rain totals from isolated storms.
Earlier on Sunday, State Emergency Service acting superintendent Joshua Clark said there had been more than 650 calls for help and 27 flood rescues in NSW during the past 24 hours.
The SES responded to 13 of those rescues across the Hunter and Central Coast.
Although the threat has been downgraded in both areas, a number of properties are isolated and landslips have occurred.
"Most of the flood rescues were for vehicles that had driven into floodwater, a few were very close to being swept away and also we've seen a couple of medical evacuations," the SES's Jake Hoppe said.
Further south in the Illawarra region, a driver had to be rescued from the roof of his car after becoming trapped by floodwaters on a causeway at the Hacking River at Otford.
Emergency services used a heavy tanker to lift the man to safety.
Matt Reeves from the Illawarra branch of the Rural Fire Service said the man was fortunate bollards along the road stopped the car from being swept away.
Western NSW in focus
The SES has also been focused on the western NSW towns of Bathurst, Mudgee and Dubbo.
On Sunday morning, Dubbo's CBD car parks, shopping centres, playgrounds, bridges, roads and campsites were underwater.
There is an evacuation order in place for Dubbo's Western Plains Tourist Park.
Flash flooding prompted dozens of calls for help at Mudgee and caused a landslip on the Castlereagh Highway, which closed a section linking Mudgee and Lithgow.
Upstream of Mudgee, the Windamere Dam has spilled for the first time since 1990 but access to the area is closed due to a landslide.
Nearby emergency services struggled to get to two people at Rylstone who were swept away in floodwaters but have since been saved.
Concerns for school holiday traffic
Moderate flooding is occurring along the Macquarie River at Bathurst, where river levels peaked Sunday afternoon.
Roads in and around the city have been impacted.
The SES is concerned about the timing of this emergency situation, given tens of thousands of Bathurst 1000 fans will be attempting to leave the city after Sunday's main race at the same time travellers make their way home after school holidays.
