The Woronora Dam catchment area has received more than 60mm of rain since Tuesday.
Downpours in the Sydney region over the past two days have exceeded the monthly average for September.
At Sydney Airport 66.6mm fell across Tuesday and Wednesday. Bankstown received 71.6mm for the same period while Holsworthy received 65.8mm.
At Warragamba, the primary dam for Sydney's water supply, 16mm of rain fell.
And with the ground wet and more rain forecast for the weekend there are hopes of modest flows into Sydney's dwindling catchments.
Moderate rainfall has fallen in the water supply catchments overnight, particularly in the Woronora and Blue Mountains catchments. There has been some minor increases in river flows. Stay updated at https://t.co/SAWOvnPRCw#WaterNSWinfo#NSWdrought@SydneyWaterNewspic.twitter.com/PrjAPdRaWT— WaterNSW (@WaterNSW) September 18, 2019
A spokesman for WaterNSW said the falls were a "favourable trend" but not enough to significantly impact water levels in the dams at this stage.
The total greater Sydney dam system on Thursday recorded a "tiny" 0.4 per cent increase in the water level since Monday, which nevertheless represents "a reversal of recent fortunes".
"But the catchment has certainly received some modest falls and that increases the likelihood of inflows if we get more significant follow-up rain," he said.
The water levels will continue to rise slowly over the coming 24 to 48 hours, the spokesman said.
Weatherzone meteorologist Graeme Brittain said the trough producing the current showers had managed to send a bit more rain further west than other recent rain events that were limited to Sydney's coastal fringe because it was a "more extensive system" producing more moisture.
More rain is forecast for the weekend although it's not expected to be as heavy as this week's showers.
The trough is set to weaken and move further offshore on Friday, which is expected to be dry, but another front and trough developing in the west are forecast to bring warm, windy weather followed by more rain and a cold change as it moves across the state on Saturday.