Sydney braces for severe weather and possible flooding as parts of the Illawarra region feel the brunt of the heavy rainfall.

In an updated warning, the Bureau of Meteorology (BOM) said rainfall that could lead to flash flooding was forecast from Saturday for the Sydney Metropolitan area, the Illawarra parts of the Hunter and Central Tablelands.

Sunday could see more of the same with possible "six-hourly rainfall totals between 70 to 120mm".

However, while Sydney is expected to be hit later this afternoon, Kiama on the state's south coast has already received 138mm of rain in the past 22 hours — exceeding its July average of 92.3mm.

More than 150mm has fallen on Wollongong, 174mm at Ulladulla and 205mm at Broughton Creek, just south of Kiama, during the same period.

The increased rain in terrain heavy with moisture adds to the potential for landslides, the BOM has also warned.

The State Emergency Service (SES) has rescued three people from rising floodwaters in the Illawarra region, two at Foxground and another at Albion Park, where more than 190mm of rain fell overnight. 

SES superintendent Tom Jory said the drivers were caught by flash flooding.

"We've seen one where a person has attempted to cross a flooded road," Mr Jory said.

"And we have seen another one where a couple of people have identified that they have become isolated as a low-level crossing has seen a rise of water over the top of it.

"They made the right call, the safe call, and they have called for help before they tried to make their own way across."

In the past 24 hours, 233 jobs have been logged with the SES across the state, most for help with sandbagging and leaking roofs. 

Federal Emergency Management Minister Senator Murray Watt said the regions copping the most rain have already seen flooding this year and the Defence Force was on standby to help.

"The NSW government have requested that we provide two night-time Defence Force helicopters with special equipment particularly for potential rescues and they have also requested that we provide 100 troops," he said.

"The helicopters will be ready from this evening, the troops will be ready from tomorrow morning.

"We all hope these resources won't be needed." 

David Williamson, his partner Michelle and their two sons were out in the rain at Sydney Harbour this morning.

"We're from Melbourne on a holiday. I think Sydney rains more than Melbourne these days," Mr Williamson said.

"I think there's 100 per cent [chance of] rain for the five days we're here," Ms Williamson added.

Catchments from the Hunter to the South Coast are also on flood watch.

The BOM says minor to moderate flooding could occur in the Hunter, Central Coast, Greater Sydney and South Coast.

While the Hawkesbury-Nepean Valley could experience major flooding. 

Some of the areas of concern include: Gosford, Sydney, Penrith, Parramatta, Wollongong, Nowra, Bowral, Campbelltown, Katoomba, Huskisson, Springwood and Bilpin.

The severe weather is expected to last until Monday as a result of an east coast low forming within the trough on Sunday.