Bureau meteorologist Helen Reid told Yahoo News Australia the soaking won't ease until Wednesday, and showers will still be lingering in parts of NSW until the end of the week.
"The rainfall is expected to be all through NSW today, but tomorrow we can see it clearing from the western part of the state," she said.
"The east coast won't see much clearance until Wednesday... it will feel like it's cleared in comparison but there will still be one or two showers lurking around. Today and tomorrow will be the big days."
Head of Resilience NSW Shane Fitzsimmons also warned the "extraordinary wet-weather event" was not over yet, with significant rainfall expected in the days to come.
"We have still got days of this extraordinary event to roll out and communities will be threatened and compromised for a while yet," he told 2BG radio.
The SES is warning more evacuations are likely as the deluge continues while Federal Emergency Management Minister David Littleproud said the situation "could get a lot worse".
Three NSW regions of most concern
Thousands of people have been evacuated on the NSW Mid North Coast and western Sydney, as rivers flood and unrelenting torrential rain continues to lash much of the state's east coast.
The Mid North Coast, western NSW and the Hawkesbury Valley are the main areas of concern on Monday with severe weather warnings in place.
There is a warning of heavy rain likely leading to flash flooding in the Mid North Coast, with BoM adding residents are at serious risk.
Roads are likely to be cut off by water and there is an increased risk of landslips. The BoM warns weather in parts of the Mid North Coast and Northern Rivers could be life-threatening.
Damaging winds about 60 to 70lm/h with peak gusts of 90km/h are also forecast along the coastal fringe just south of Seal Rocks from Tuesday morning.
From 9am Sunday to 4am Monday, Minnie Water, about 50 kilometres from Grafton, recorded rainfall totals of 241mm.
Yarras Mountain copped 202mm while Kempsey was doused by 173mm.
NSW flood evacuation orders
Evacuation orders are now in place for low lying areas of Kempsey, Macksville, Port Macquarie, the lower Macleay, Wauchope and Rawdon Island, Taree and Wingham.
Ms Reid told Yahoo News Australia some parts of the Mid North Coast will be doused by 150mm of rainfall on Monday and inland areas will see at least 100mm, with heavy rain and flash flooding warnings in western NSW.
The widespread rainfall hitting the west could lead to riverine flooding in Tamworth, Dubbo, Tibooburra, Cobar, Orange, Moree and Bourke.
Heavy rainfall from western areas will also extend to parts of the ACT, South Coast, Snowy Mountains and Northern Tablelands on Tuesday.
Communities along the Hawkesbury River on the outskirts of Sydney are also bracing for once-in-a-generation flooding that could displace thousands of residents and disrupt utilities for months.
Hawkesbury area expecting worse flooding in 60 years
Major flooding is occurring along the Hawkesbury River at North Richmond and Windsor in Greater Sydney, with major flooding expected at Sackville and downstream.
The BoM is warning the Nepean River at Penrith is currently at 8.03 metres and is expected to rise as rainfall continues.
The Hawkesbury River at North Richmond is sitting at 13.16 metres and could also increase as rainfall lashes the area.
There are concerns the Hawkesbury River at Windsor may peak near 13 metres on Monday evening, a level similar to flooding events that occurred in April 1988 and July 1990.
The Hawkesbury River at Sackville could also peak at the major flood level of 9.7 metres on Monday and into Tuesday, also a similar level seen during the flooding event in 1990.
The Hawkesbury is predicted to reach peaks of up to 15 metres and the SES says homes and properties will be flooded, some up to roof height.
The BOM expects the worst flooding event to the area northwest of Sydney since November 1961.
Evacuation orders have been issued in western Sydney, with NSW SES urging residents in Eastern Creek to evacuate by 3am and in Windsor by 9am Monday.
Parts of Penrith, Jamiestown, Cornwallis, Freemans Reach and Pitt Town are among the areas ordered to evacuate.
Ms Reid said the weather event sweeping across NSW currently was "up there with the best of them".
NSW flooding a 'nightmare rollercoaster'
Communities still recovering from the black summer bushfires and the impacts of COVID-19 have been dealt another devastating blow, with homes washed away, livelihoods ruined and thousands evacuated amid record-breaking floods.
Much of the Mid North Coast that was ablaze at the end of 2019 is now underwater.
Port Macquarie restaurateur Nathan Tomkins says the past few days have been a nightmare rollercoaster.
After record flooding at the Hastings River over the weekend, Mr Tomkins' restaurant was inundated with neck-high water.
After surveying the damage on Sunday he says the venture he's spent 24 years building is in ruins.
"The water just went right through and just destroyed everything. There is nothing left," he told ABC TV on Monday.
"This is just like a nightmare, it really is. I've got so many friends and family members that are just hurting at the moment from all this, who have lost their homes, lost their cars, lost their businesses."
The nightmare isn't over yet either, with much of the Mid North Coast bracing for more flooding on Monday.
"I'm feeling like I'm on a roller-coaster. I woke up this morning and I just pinch myself to go, 'Okay, this is not real'. But it's real," he said.
Hundreds of homes have been damaged, including one filmed floating down the Manning River in Taree, but the Emergency Services Minister says it is only a matter of time until lives are lost too.
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