Greater Sydney's Hawkesbury Nepean Valley - incorporating major urban centre Penrith - is set to cop its worst flooding in 60 years as people around NSW are forced from their homes by incessant rain.
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The state's emergency service, meanwhile, says it will be working beyond Easter on the post-flood clean-up effort and restoration of key services.
The heavy rain kept falling overnight and into Sunday morning as rivers across NSW and near Sydney overflowed or threatened to flood.
Residents in Pitt Town Bottoms, Pitt Town North, Cornwallis, North Richmond, Grono's Point, Freemans Reach and Agnes Banks west of Sydney were told to evacuate in the early hours as the Hawkesbury River flooded.
The Bureau of Meteorology's Justin Robinson said forecasters believed the Hawkesbury Nepean Valley would experience its worst flooding since 1961, with the spilling of a full Warragamba Dam prompting major concern.
Warragamba was hit by more than 150mm of rain in the 24 hours to 9am on Sunday, and more than 250mm over the past four days.
People in low-lying areas in the Hawkesbury Nepean Valley have been told to protect homes by sandbagging doorways and clearing drains.
The Nepean River at Penrith could rise as high as 10 metres by 9pm, while the river at Richmond and Windsor would not peak until Monday.
Mr Robinson said major flooding was already occurring at North Richmond.
Moderate flooding also continues along the Colo River, with farmers near the river told to be on alert and ready to move livestock.
"It is one of the biggest floods we are likely to see for a very long time ... floodwaters at Penrith are expected to then move downstream and impact those communities at North Richmond, Windsor, Sackville," Mr Robinson said.
Elsewhere, the Sydney CBD was drenched by 110mm of rain over the same 24-hour period, while 120mm hit Hornsby and 168mm reached Katoomba.
The bureau's Agata Imielska said the severity of rain hitting Greater Sydney would ease from Sunday night, but the mid north coast would continue to be drenched and inland NSW would be deluged from Monday.
Ms Imielska said the NSW northwest slopes and plains would receive four times more rain in two days than the entire March monthly average.
The NSW south coast would also experience heavy rain from Tuesday, with little respite for any region across the state until Wednesday.
SES Deputy Commissioner Daniel Austin told the ABC the service was dealing with downed trees, power outages, clearing of debris and damage to houses.
The SES has responded to almost 7000 calls for help since Thursday.
"We're planning well beyond Easter for our own operations," Mr Austin said.
Premier Gladys Berejiklian on Sunday said that the Hawkesbury Nepean Valley flood could prompt an additional 4000 evacuations by the day's end.
The federal government's natural disaster arrangements have been activated for a large stretch of NSW from the Central Coast to Tenterfield.
"We are envisaging a one-in-50-year event, yesterday we were hoping it would only be a one-in-20-year event," Ms Berejiklian told reporters.
The rain and foul weather is being caused by a coastal low-pressure trough combined with a strong high pressure system in the south.
Bellingen locals on NSW's mid-north coast and people at a tourist park were advised to evacuate because of the risk of flooding along the Bellinger river.
Parts of Port Macquarie, Taree and nearby towns have also flooded in what Ms Berejiklian on Sunday labelled a "one-in-100-year event" on the mid-north coast.
Meanwhile, a bodyboarder in his 60s went missing off the Coffs Harbour coast on Saturday afternoon and crews will resume the search on Sunday.
Strong winds have also caused damage, with a small tornado ripping through Chester Hill High School in Sydney's west on Saturday.
Both the South Australia and Queensland governments on Sunday announced they would send rescue teams to NSW for emergency assistance.
Australian Associated Press