The Bureau of Meteorology says the heaviest falls will hit the Illawarra, Hunter, Central Coast and Greater Sydney from Saturday evening into Sunday.
Wollongong in the Illawarra, the eastern suburbs of Sydney and up to Newcastle in the Hunter could get up to 100mm while other areas will have between 30mm and 50mm.
Most areas will not experience a sudden downpour with the rainfall totals to accumulate over the next 48 hours.
"So even if it's only taking up eight or 10mm an hour, if we did that for several hours in a row, that's still a lot of rainfall by the time of day," senior meteorologist Angus Hines told AAP.
Mr Hines warned the rainfall may lead to flooding as the east coast is already saturated after a dumping in April.
"This is significant rainfall that could lead to flooding, perhaps localised flooding, certainly blocking terrain, closed roads and traffic jams," he said.
The deluge is far less than a month ago when floodwater reached the top of cars in some areas of the Illawarra and Sydney.
"So we're not expecting to see the same type of flood response, but it's enough rain that we will probably see some areas of localised flooding and Illawarra is again in the firing line," Mr Hines said.
A wet weekend ahead for — Bureau of Meteorology, Australia (@BOM_au) #NSW as rain with possible storms spreads across the state. Saturday night and Sunday look to be particularly wet for coastal locations including Sydney. Areas of flooding and travel impacts possible. Latest: https://t.co/jlOoTZL1iF pic.twitter.com/E4VBz5duUgMay 3, 2024
The Sydney Half Marathon is set to go ahead on Sunday with organisers monitoring the conditions, and more than 19,500 runners registered.
"At this stage there are no indications that any changes will need to be made to the event," a spokesman told AAP.
"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."
The conditions are expected to ease on Monday although rain will continue to pepper the east coast through next week.
The NSW State Emergency Service is closely monitoring forecasts and preparing assets and personnel for potential issues.
SES Zone Commander Allison Flaxman warned NSW residents to prepare for the wet weather and once the heavier rain hits to avoid heading out onto the roads.
"If you do need to go out in this weather, if you do come across any flooded roadways, please don't drive through them," she told the Nine Network.
The weekend falls come after a wet week for NSW. Rainfall data from the bureau showed Observatory Hill had nearly 53mm from Monday to Friday and some areas had totals for the week of more than 100 mm.
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