The NSW State Emergency Service (SES) is directing people in several areas on the Mid-North Coast to evacuate before 8:00pm, as the region is battered by wild weather and heavy rain.

It says floodwaters will inundate a "significant number of properties" in low-lying areas of Port Macquarie when the Hastings River level reaches 1.75 metres.

Evacuation orders have also been issued for low-lying areas of:

• North Haven
• Dunbogan
• Diamond Head
• Laurieton
• Kempsey

"Once flood water begins inundating the area road access water, sewerage, power, phones and internet may be lost. If you remain in the area you will be trapped and it may be too dangerous to rescue you," the SES said.

The Bureau of Meteorology (BOM) has also issued a severe thunder warning for the Northern Rivers and parts of the Northern Tablelands.

"Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding in the warning area over the next several hours. Locations which may be affected include Lismore, Grafton, Tenterfield, Kyogle, Yamba and Maclean," the BOM said.

Meanwhile, Sydneysiders have been urged to stay indoors on Saturday as "dangerous and threatening" rain is expected to cause flash flooding.

Up to 100 millimetres of rain is expected to hit the city as the weather system, which has already caused significant flooding on the Mid-North Coast, tracks south.

The BOM's Agata Imielska said the falls in Sydney would be significantly more intense than in recent days.

"This will be the difference between what could seem like inconvenient rain, to actually something that might be quite dangerous and threatening," she said.

"It is really important for Sydneysiders to be mindful about reconsidering their plans."

The NSW Flood Operations Manager, Justin Robertson, said there were concerns about already swollen waterways throughout Greater Sydney.

"We're particularly concerned about those catchments in the broader Sydney area, and also in the Sydney metropolitan area where we might see quite significant flash flooding," he said.

"We've got a flood watch current for both the Nepean and Hawkesbury rivers, we're expecting that to be around the moderate level, but even higher flood levels are possible.

"Some of our modelling does indicate that we're likely to see a spill from Warragamba Dam and that will then impact those communities downstream."

Ms Imielska said there was "very heavy, potentially life-threatening rainfall happening on the Mid-North Coast".

"We're seeing rainfall totals of over 100 millimetres falling in about an hour, so that is very dangerous."

Steve Lawrence from the SES said residents should stay across alerts.

The SES volunteers last night performed 14 flood rescues as motorists became trapped on flooded roads but Mr Lawrence said that people have mostly been doing the right thing.

The rainfall is expected to continue into Sunday but will begin easing before another rain event is due to impact inland parts of the state early next week.