Two people have been killed and many others rescued after severe flooding in New South Wales, Australia.

An evacuation order has been issued for residents of Kempsey on the NSW mid-north coast as the Macleay River rises to flood the central business district.

The order, and another for low-lying parts of Port Macquarie, comes as a low pressure system moves down the NSW coast bringing high winds, flooding and heavy rain.

The Bureau of Meteorology (BoM) has predicted the Macleay River at Kempsey Bridge will peak near 7.3 metres at 6am (AEDT) on Sunday with further rises possible.


It said the levee at Kempsey would be overtopped at about 9pm on Saturday, causing inundation of the CBD.

The State Emergency Service (SES) issued a warning to CBD residents not to delay their evacuation as roads could be congested or closed.

"You could become trapped and need rescue. Remaining in flooded areas is dangerous and may place your life at risk."

The SES urged affected residents to go to friends or relatives or evacuation centres that would be set-up at Kempsey High School.

An evacuation order has also been issued for low-lying parts of the Port Macquarie CBD, Settlement Point and North Shore areas, Hibbard Drive, Fernbank/Oaks Creek and low lying parts of Westport.

The BoM predicted the Hastings River will peak at 1.9m at the Settlement Point gauge at 10pm on Saturday with further rises possible.

A man's body was found in a submerged car on the NSW north coast, prompting police to urge motorists not to drive into floodwaters.

SES volunteers were called to Rogan Bridge Road at Mylneford, about 20km northwest of Grafton, shortly before 1pm (AEDT) on Saturday after a car was seen submerged in floodwaters.

They found the body inside the car and alerted police who are working to identify the man.

Police are urging motorists not to drive into floodwaters following the man's death and the rescues of at least 15 other people from floodwaters during the severe weather passing down the NSW coast.

The man's death follows that of a 17-year-old boy who was sucked into a drainpipe on a golf course on the mid-north coast on Friday night.

The boy was collecting golf balls in waist-deep water with friends in the NSW town of Kew before he got swept away.

A friend found his body on Saturday morning.

A rescue helicopter has flown a newborn baby and other children to safety from a farm cut off by floodwaters on the NSW mid-north coast.

The Westpac Lifesaver Helicopter was returning to pick up other family members at Mount Seaview Reserve on Saturday afternoon when it was diverted to rescue a woman trapped in her car by floodwaters.

Helicopter spokesman Steve Leahy told AAP that rescue got underway at around 4.30pm (AEDT).

He said a family of up to eight needed to be evacuated from their farm about 30km west of Wauchope and the helicopter on its first trip had flown the baby and other children to Port Macquarie.

"The farm was isolated by floodwaters and the newborn baby was a little bit sick or in distress."

More than 3850 people are isolated on the north coast and the NSW SES has received more than 1570 calls for assistance.

No major towns have been cut off, but the SES holds concerns for the NSW town of Bellingen.

"At this stage we haven't had any reports of any major inundations but we're still keeping a close watch on that area," an SES spokeswoman told AAP on Saturday.

Overnight the SES had to perform 15 flood rescues, most of which they say could have been avoided.

"Many of these rescues relate to deliberate entry into floodwater and the NSW SES urges people not to drive, ride or walk through floodwater," it said in a statement.

Flood warnings have been issued for areas around 17 NSW rivers, with major flood warnings in place for the Clarence River and the Bellinger River.

A moderate to major flood warning has been issued for the Macleay River and residents in the Lower Macleay region have been ordered to evacuate.

"We've been door-knocking in the area," the SES spokeswoman said.

"At the end of the day we can't force them to, but we're asking that they leave immediately.

"Once we door-knock it's up to them to go."

A severe weather warning remains in place for the mid-north coast, Hunter, Illawarra, Sydney and parts of the central tablelands.

Heavy rain that could lead to flash flooding is predicted for those areas throughout Saturday.

Sydney will experience winds of about 50 to 65 km per hour on Saturday evening.

"Along the coast winds are quite strong," a BoM spokesman told AAP on Saturday.

"There's also some quite large seas and swell so the surf is quite dangerous and rough."

"It's generally pretty wild and woolly out there."

The SES has received about 70 calls from the Sydney metropolitan area but says there hasn't been too much damage around the city.

Cronulla's Wanda Surf Club did lose part of its roof in the wild weather on Saturday.

The BoM says the low pressure system should move south on Sunday morning, easing weather conditions around Sydney.

The Transport Management Centre has advised NSW residents to avoid travelling on the state's north coast as severe weather has caused road closures.

The Pacific Highway is closed near Macksville and at South Grafton.