Parts of NSW, including Sydney, face a battering from wild winds just days after deluges caused major damage and drove mass evacuations.

With hundreds of people isolated and rail lines still down on Tuesday after heavy rain at the weekend, warnings were issued for strong winds and hazardous surf across eastern NSW.

“Severe thunderstorms are possible over eastern NSW on Tuesday afternoon and evening, with damaging winds the main threat,” forecaster Weatherzone wrote.

“Wind has also been picking up on Tuesday as the change moves its way up the NSW coast. These southerly winds will strengthen further into Wednesday as a low-pressure system deepens over the Tasman Sea.”

Damaging southerly wind gusts of up to 100km/h are possible along parts of the NSW coast between about Sydney and Coffs Harbour on Wednesday, Weatherzone said. The greatest risk of damaging gusts for the Sydney and the Hunter coasts will be in the morning, while the risk continues into the afternoon for the mid-north coast.

Coastal erosion from very heavy surf is also possible at south-facing beaches between Sydney and Sea Rocks from Wednesday morning, particularly around high tide.

The Bureau of Meteorology has issued a warning for hazardous surf for the east coast north of Ulladulla. Likely affected locations include Sydney coast, Coffs Harbour, Port Macquarie, Newcastle, Woolgoolga, Sawtell and The Entrance.

There is a similar warning for the southern mid-north coast, Hunter and Sydney metropolitan forecast districts from early Wednesday morning.

“Beach conditions in these areas could be dangerous and people should stay well away from the surf and surf exposed areas,” the BOM said.

The same front brought the first snowfalls of the year to Australia’s mainland ski resorts on Tuesday.

It came as about 800 residents returned to homes north-west of Sydney and along the Hawkesbury River after evacuation orders from the weekend were lifted. The NSW State Emergency Service is still advising people to avoid the area.

Flood levels at the Hawkesbury and Nepean rivers were significantly reduced by Tuesday morning. The Hawkesbury River reached its peak of 10.52 metres – about seven metres above average – at 9pm on Saturday.

Parts of the south coast rail line remain out of operation with significant damage to tracks.

Meanwhile, bulldozers will cut a temporary evacuation road in the Blue Mountains’ Megalong Valley after a land slip shut off the only access road, isolating 150 residents and 200 tourists.

Blue Mountains mayor Mark Greenhill said cutting the road through rock would take three days and would be contingent on careful geotechnical advice, which was expected on Tuesday.

“[It] will take about three to four days to do, it’s a lot of work but our teams are ready to go. We’re just waiting for approval from the geo-technicians, we want it to be done safety,” he told the ABC.

Hundreds of residents are cut off in the Blue Mountains after this slip took out their access road. Photo: AAP

Food, fuel and essential supplies have been dropped into the valley by helicopter. All visitors have been evacuated via another route over private land.

Elsewhere, the NSW government is weighing up reducing storage levels in Sydney’s Warragamba Dam after the weekend’s heavy rain.

Councils along the Hawkesbury have reignited calls for the dam wall to be raised, a plan promised by the former Coalition government but abandoned under Premier Chris Minns’ Labor administration.

Sydney’s largest reservoir has spilled a total volume of water equivalent to half of Sydney Harbour since it reached 100 per cent capacity on Saturday, according to Water NSW.

Minns said rebuilding the dam wall would take eight to 10 years, was expensive and would not stop flash-flooding in western and north-western Sydney.

“Forty-five per cent of floodwaters in the Hawkesbury, Richmond catchment don’t come over the top of Warragamba Dam, so we could be in a situation where we raise Warragamba, we spend $2 billion and those communities are still inundated by flooding,” he said.

Tweet from @TransportforNSW

Minns said the government would look at all potential changes, including reducing the maximum allowable water level at Warragamba, as long as Sydney’s drinking water supply could be supplemented.

“That’s not going to give us an immediate relief, it’s going to take a bit of time,” he said.

Hawkesbury mayor Sarah McMahon said more needed to be done to ensure her community was not left devastated by more flooding.

“Nobody has seen a ‘plan’ and won’t see a ‘plan’ for at least another year, and who knows how many more floods will have hit us before we have this ‘plan’,” she said on Monday.

Disaster assistance has been made available for affected residents and councils across the state.

-with AAP