Parts of Sydney have been issued a dire warning and have been told to brace for the worst flooding in almost 60 years.
The Bureau of Meteorology's Justin Robinson said it was expected river levels in Penrith would be comparable to those seen in the 1961 floods after a combination of heavy rain that fell in the area on Saturday night and Warragamba Dam spilling.
"We are very concerned about the flooding in the Hawkesbury-Nepean Valley," he said at a press conference on Sunday.
"To give you some context... that is bigger than the February 2020 flood.
"It is bigger than the 1988. It is bigger than the 1990 flood, and it is bigger than the 1964 flood. It is one of the biggest floods we are likely to see for a very long time."
Flood waters in Penrith may impact communities in North Richmond, Windsor, Sackville and further downstream, and those in the area must be aware of any potential evacuations and advice from authorities.
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.
Mr Robinson implored people in those areas to people to stay updated regarding the "dangerous" situation as it evolves over the coming days.
NSW SES Assistant Commissioner Dean Storey said if you are subject to an evacuation warning, you need to act now.
"That means you need to prepare yourselves and your families and your pets and animals, right now for a potential evacuation," he said.
"You may choose to self evacuate, that would not be a bad decision if you have that option available to you.
"If where you are residing is subject to an evacuation order you must leave immediately. Do not delay, leave immediately, follow the advice and direction of authorities on the ground, they will tell you what you need to go and what options you have."
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.
Sydney's CBD was drenched by 110mm of rain over the same 24-hour period, while 120mm hit Hornsby and 168mm reached Katoomba.
Oakdale and Picton to Sydney's southwest received more than 180mm.
BoM'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 inundated from Monday.
She said the NSW northwest slopes and plains would receive four times more rain in two days than the entire March monthly average.
Record-breaking floods declared as natural disaster
Speaking to reporters on Sunday, NSW Premier Gladys Berejiklian said the current situation was a one-in-50-year event for those residing in the Hawkesbury-Nepean area.
Ms Berejiklian said the Mid North Coast was also experiencing a rare weather event.
"Regrettably, parts of the Mid North Coast are experiencing a one-in-100-year event, and while we don't think things will worsen on the Mid North Coast, definitely conditions will continue," she said.
The premier once again urged people not to leave their homes unless they have to.
"We ask that everybody be extra cautious, this is not a time to be out and about unless you have to be," she said.
Ms Berejiklian confirmed Victoria, South Australia and Queensland have provided assistance for the flood rescue.
The federal government's natural disaster arrangements have been activated for a large stretch of NSW from the Central Coast to Tenterfield.
On Saturday, Police and Emergency Services Minister David Elliot said he had spoken with Federal Agriculture, Drought and Emergency Management Minister David Littleproud and signed off on 16 natural disaster declarations.
"They are 16 local government areas primarily the north of the state, they will get that assistance," he said.
Mr Elliot added there were 74 flood rescues overnight and had some stern words for motorists across the states.
"Motorists in this state need to know we are statistically moving closer and closer to the inevitable fatality," he said.
"We cannot say it enough, do not put yourself in danger, do not put the agencies that are there to assist you and event of a flood rescue in danger.
"I am horrified to think that there were unnecessary flood rescues going on."
He said there will be “inevitable” rescues, as there are many houses surrounded by flood waters, but some motorists are taking “unnecessary risks”.
“The unnecessary risks that are being taken by these motorists is beginning to wear thin on the authorities,” he added.
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