"People are probably going to be heading out and about on the roads, but it's probably going to be dangerous driving conditions," weather bureau senior meteorologist Dean Narramore told AAP.
"Early next week we could see flooding of campgrounds, and things like that.
"So, a soggy start to the school holidays."
Heavy rainfall is predicted from the Illawarra to the southern part of the Mid North Coast from Saturday to Tuesday, but the precise areas of heavy impact are hard to predict until the system settles.
Mr Narramore said the heaviest falls are expected on Sunday, Monday and Tuesday, with the most impact around Port Macquarie to Batemans Bay.
In Sydney, about a month's worth of rainfall is predicted to fall in the coming five days, with about 100mm to 150mm predicted.
Mr Narramore said a more precise forecast of which areas can expect heavy rainfall will form in the coming days.
"This is one of those developing situations like we saw in March, where every day we get a closer idea of exactly where the focus will be, where the worst of the conditions will be, and it's a pretty quickly changing situation," he said.
"We'll have a good idea of where the main focus will be for that as we move into the weekend."
The falls are predicted because of an upper-level low pressure system tracking over the east, which will bring tropical moisture from the north as it becomes stationary.
"We're looking at multiple days of widespread showery, rainy conditions and multiple days of rainfall on already saturated soils and catchments and full dams," Mr Narramore said.
He said this could result in quick or renewed river level rises, followed by flooding next week.
A warning for local to major flooding from Saturday in the Sydney metropolitan area has been issued by the NSW State Emergency Service.
The service is predicting minor to major flooding on the Upper Nepean, Lower Nepean, Hawkesbury and Colo rivers, while minor flooding is expected for the Cooks, Georges and Woronora rivers.
Local flooding is expected at the Upper Coxs, Macdonald, Northern Sydney, Southern Sydney and Parramatta rivers.
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