Hundreds of millimetres of rain is set to lash Sydney and the surrounding regions, with NSW facing the most torrential downpour since the late 1990s.

Flash flooding, monster waves and high winds are expected to batter NSW over the weekend as emergency services brace for the worst.

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The Bureau of Meteorology said about 100mm of rain is expected across parts of the state on Saturday - and double that could fall on Sunday - because of an east coast low.

“The rainfall will be variable but hundreds of millimetres. It’s not something we see that often,” BOM acting NSW state manager Jane Golding.

“The multi-day rainfall from this event looks like much more than what we had during the June 2016 east coast low which was our last significant east coast low.

“Potentially we haven’t seen anything like this since the late 1990s.”

Rescuing people

More than a dozen people have had to be rescued after trying to drive through floodwaters, with the SES issuing a fresh warning to motorists ahead of more heavy rain.

The SES received about 1600 calls or requests for assistance on Friday and rescued 13 people from floodwaters on the NSW Central Coast.

NSW SES Deputy Commissioner Daniel Austin told Weekend Sunrise on Saturday people shouldn’t drive through floodwater.

“It is dangerous. It is one of those things that you just can’t assess before you get there so you need to stay away from it,” he siad.

“We’ve seen many examples over the years where we’ve seen people playing around near floodwater and were asking people to really stay away from that.

“Don’t expose yourself and don’t expose the rescuers who then need to come and assist you.”

The SES warning comes as the Bureau of Meteorology forecast more wild weather and flooding across NSW on Saturday.

Rain bucketed down on the Central Coast overnight with Mt Elliot (184mm), Toukley (159mm), Kangy Angy (140mm) and Wyong (125mm) receiving heavy falls from 9pm Friday to 4am Saturday.

A trough lying along central and northern NSW is expected to bring heavy rain and wind to Sydney and the surrounding regions on Saturday before extending to the South Coast late Saturday and Sunday.

The BOM has warnings in place for the Brunswick, upper Nepean, Orara, Moruya, Clyde and Colo rivers, Wollembi Creek, Tuggerah Lake and St Georges Basin, with moderate to major flooding expected.

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Strong winds were also forecast, with waves of between five to six metres set to batter the coast between Batemans Bay and the Central Coast.

After being ravaged by recent fires, the state’s south coast is now set to be lashed by heavy rain from late Saturday.

The BOM warned that the burned out landscape and loss of vegetation had left it vulnerable to landslips.