The Illawarra's dams are expected to fill - and possibly overflow - in the coming days, after almost 400mm of rain fell on one day in some parts of the region.
According to WaterNSW, two of the dams in the Illawarra and Sydney water catchment are completely full and overflowing.
At midday on Monday, the total Greater Sydney Catchment was almost two-thirds full, a massive jump from the 42.7 per cent storage recorded a week ago.
"Warragamba Dam is up from 42% of capacity last week at the outset of the rain event that swept the state's east, to 60.7% and continuing to rise," a spokesman from WaterNSW said.
"All of Sydney's other dams that make up the Greater Sydney supply network are expected to fill in coming days, with Nepean and Tallowa Dams already spilling."
According to WaterNSW data, 44 per cent of the rain which fell in the Nepean Catchment in the last year has fallen in the past week.
And in the 24 hours to Monday morning, that catchment - which includes all the dams west of the Illawarra - received 22 per cent of its rainfall total for the past year.
The second largest dam in the Sydney catchment, Cataract Dam, which was at 26 per cent storage a week ago, was 68.1 per cent full.
At the SES headquarters in Wollongong, Premier Gladys Berejiklian said it was too soon to know if the increased dam levels would make any difference to current water restrictions.
"There's no doubt that given the extreme weather conditions that we're seeing this summer, whether it's the horror and devastation of the fires, whether it's the devastation caused by torrential rain, we need to be aware that extreme weather conditions means we need to be cautious about how we use precious commodities like water," she said.
"I don't want to speculate on what we might do until the current weather fronts have gone through NSW and we assess the situation properly."
She also said there was "nothing to suggest we should change course" in terms of ramping up the operations of Sydney's desalination plant, as the state needed to plan for the future.
Avon Dam was at 80.6 per cent (up from 44.5 per cent) and Cordeaux was at 68 per cent (up from 36.5 per cent)
In the 24 hours to 9am on Monday, Robertson was the rainiest place in the state, receiving just shy of 400mm of rain.
According to the Bureau of Meteorology's monitoring stations, in the same period Macquarie Pass got 362mm, Dombarton got 255mm and Albion Park Post Office got 192mm.
Darkes Forest got 172mm, Bellambi got 137mm Kiama got 110mm and Rixons Pass (Bulli) got 102mm.