The Bureau of Meteorology says the Illawarra, Hunter, Central Coast and Greater Sydney regions can expect to get a similar triple-digit dumping over Saturday night and Sunday.
"We can see some pretty major impacts, areas of flooding, blocked drains, closed roads, causing big traffic. We could see all that on Sunday," Senior Meteorologist Angus Hines said.
Rainfall data from the bureau showed Observatory Hill had nearly 53mm from Monday to Friday and some areas had rain totals for the week of more than 100 mm.
The Sydney Half Marathon is set to go ahead on Sunday despite the conditions, with more than 19,500 runners registered.
Organisers told AAP they were continuing to monitor the situation.
"At this stage there are no indications that any changes will need to be made to the event," a spokesman said.
"Athletes are encouraged to prepare for a wet race and run to the conditions and the team is confident the event will take place as planned."
Thunderstorm warnings are in place in the central and northern inland areas of the state with slow-moving storms expected to bring localised heavy falls and a risk of flash flooding.
The NSW State Emergency Service is closely monitoring forecasts and preparing assets and personnel for potential issues.
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