The city is on track to exceed the previous annual record of 2194mm of rain by the weekend.
Bureau of Meteorology senior forecaster Jonathan Howe says Sydney is just 27mm away from the 1950 benchmark.
"We have seen some pretty isolated falls above 50mm across NSW but the heaviest falls are along the coast," he told ABC TV on Thursday.
The deluge will see inland rivers on and west of the Great Dividing Range hit by moderate to major flooding affecting towns such as Tamworth, Dubbo and Bathurst.
There is a severe weather warning for heavy rain and clusters of thunderstorms across the state's inland, with no reprieve in sight until next week.
Much of the state will see showers and isolated thunderstorms on Thursday afternoon, with some areas over the southwest set for severe storms, heavy rainfall, possible hail and damaging winds.
Assistant SES commander Sean Kearns said there were more than 40 warnings out across the state, with particular concerns about flooding in the communities of Forbes, Warren, Dubbo, Nyngan and Bathurst.
"We are also watching this weather system as it may move across metropolitan areas on the east coast," he told Nine's Today program.
Flood Recovery Minister Steph Cooke said there were flooding threats across the state, with more than 500 SES volunteers working in the field.
"Our landscapes are entirely saturated, our dams are basically full and the river systems, those channels, are also full," she said.
"Any additional rain, even the smallest amount, vastly increases the risk of both riverine but particularly flash flooding given the nature of the weather system that's coming through."
A warning area for severe weather extends from the Queensland border in the north to Wilcannia in the state's central west.
It comes as many NSW towns are dealing with renewed flooding, including in the small cotton and wool town of Warren.
"It hasn't been hammering with rain, but we have had such a lot of rain consistently, basically since Christmas, that it just doesn't have a chance to dry out," Macquarie Caravan Park owner Carolyn Monkley told AAP.
"Rainfall here is totally different to rainfall on the coast. A couple of inches out here makes it very very, very soggy."
Other towns of particular concern include Condobolin, Nyngan, Cobar, Bourke, Wanaaring and Brewarrina.
Flood warnings are current across inland catchments including the Namoi, Macquarie, Bogan, Lachlan, Murrumbidgee, Murray, Edward, Culgoa, Birrie, Bokhara, Warrego, Paroo, Barwon, Darling, Macintyre and Snowy rivers.
A flood watch has also been issued for coastal catchments including the Hunter, Hawkesbury and Colo rivers, and Wollombi Brook, with rises expected from Thursday to Friday.
The federal government has announced disaster assistance will be extended to 27 local government areas in inland NSW.
The funding can help people whose homes or properties have been damaged in floods, help councils with restoring essential assets, or be used by primary producers or small business owners.
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