Destructive thunderstorms with large hailstones are headed for parts of NSW, as the search continues for a teenager missing in floodwaters in southwest NSW.

Emergency crews and NSW Police are searching for a 19-year-old man who went missing around the flooded Murrumbidgee River near the Victorian border town of Balranald on Saturday.

Conditions remain hazardous at Balranald as the Murrumbidgee continues to rise on Monday, with an expected peak of 7.4 metres this weekend.

It comes as a severe storm warning was issued for the NSW Mid North Coast through to the New England region in the west on Monday.

A trough and unstable airmass created the severe thunderstorms, which could lead to large hail, heavy rainfall, flash flooding and damaging winds on Monday afternoon.

The warning area includes Port Macquarie on the coast, Kempsey and Comboyne, and extends west to Walcha, Uralla, Armidale and Barraba.

It follows severe storms battering parts of Sydney, the Central Coast and Wollongong on Sunday night.

The SES received 100 calls for help in the 24-hours to Monday morning – 30 of them in the Sydney metropolitan area – mostly for trees or branches falling on homes and driveways.

There are 68 flood warnings across the state – 12 of them at emergency level.

NSW today entered the 75th day of the ongoing flood crisis, as the SES remains focused on two towns in western NSW, where the swollen Lachlan River has cut access to both Euabalong and Moulamein.

Locals in Moulamein have been cut off for more than a week and flood waters are yet to reach their peak, NSW SES Southern Zone Incident Controller Ben Pickup said.

“We are monitoring conditions closely, and fortunately at present the levee is holding,” Mr Pickup said.

“The town has been cut off to vehicles, so resupplying isolated residents continues to be a major focus via helicopter and boats.”

Euabalong publican Neil Quinn said the town last week stared down the swollen river with a hastily-built levee bank holding back floodwaters.

“I don’t know how we did it, but we stopped the river,” Mr Quinn told AAP on Sunday.

It followed a tense week when the town’s original flood bank began to fail and evacuation orders were issued.

Mr Quinn said the town would be relying on helicopters for supplies for at least a month.

The SES is also focused on the towns of Hay, Balranald, Brewarrina, Bourke and downstream towns, Torrumbarry, Barham and Boundary Bend.