Sydney's Warragamba Dam will start to overflow on Friday evening, the result of a month of heavy rainfall.
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Water levels are rising even higher as Sydney cops a drenching, with up to 100mm of rain expected in the dam area.
Water NSW warns that a spill will start on Friday night, and on Saturday morning the peak rate of water over the dam may be up to 100 gigalitres per day.
Conditions across the state are expected to ease during the final weekend of a month that has delivered triple the average rainfall to some areas.
Flooding risks throughout the state will continue even after the rain stops, however.
The State Emergency Service has already responded to more than 3000 requests for assistance this month, including 600 in the 24 hours to 4pm on Friday.
They include 16 flood rescues.
On Friday morning, a man had to be rescued by an RFS helicopter at Caroona near Quirindi when he was trapped on the roof of his car in floodwaters that exceeded 1.5 metres.
Thunderstorms hit the Hunter region on Friday morning, delivering 64mm and causing major flooding at Kingdon Ponds in Scone.
Nearby Mount Palmer had the state's highest 24-hour total with 117mm.
"We've seen thunderstorms and more localised rain bringing those heavy and more intense bits of rainfall, it hasn't been necessarily raining constantly nonstop … but we still have that risk of thunderstorms," Bureau of Meteorology senior meteorologist Agata Imielska said.
She warned against complacency when the weather broke.
"Today is still a key day to keep a close eye on conditions but we will see an easing change into the weekend," Ms Imielska said, with mostly clearer conditions and occasional showers expected.
While floods threaten some areas, other are rejoicing at the rain.
Broken Hill recorded 23mm, and Ms Imielska said: "That was actually welcome news. That's the most rainfall they've had since September 2020."
Some regional and remote towns face isolation, with floods cutting off major roads including the Kamilaroi, Golden, Cobb and Silver City highways on Friday morning.
In New England, major flooding along the Namoi River has cut off the town of Wee Waa.
It is protected by a levee but could be isolated for more than a week.
Floods are threatening western Sydney after Richmond recorded more than 55mm since 2.30pm on Thursday, about half of which fell in three hours.
Minor flooding is possible from Friday afternoon in the Hawkesbury-Nepean Valley.
In the state's central west, floodwaters along the Lachlan River are coursing through Jemalong on their way to Condobolin.
Forbes residents are warned the river still poses a danger after evacuations were ordered last week.
Renewed rises upstream at Cowra on Friday morning are expected to bring minor flooding to Nanami on the way to Forbes.
The Castlereagh River has reached moderate flood levels, with Mendooran and Gilgandra expected to feel the impact.
Floodwaters have crept into Molong, with the creek in the town at moderate flood levels, and similar levels are expected along the Bell River at Wellington from midday.
Minor flooding continues along the Macquarie River at Wellington Bridge.
The river has fallen in nearby Gunnedah, but moderate flooding is still under way.
Minor flooding is occurring at Narrabri, with the river expected to exceed moderate flood levels on Friday.
There's a warning for damaging wind gusts over parts of the Snowy Mountains and southern tablelands on Friday evening.
Surf conditions are also expected to be hazardous along much of the coast.
Australian Associated Press