Wild winds batter New South Wales

Sydney Harbour Bridge. Photo / Thinkstock

Train services west of Sydney have been cancelled after winds up to 137km/h caused trees to fall on two moving trains.

The Great Western Highway through the Blue Mountains is also restricted in some sections by fallen tree branches.

More than 20,000 homes are without power in the mountains, the southern highlands and on the south coast.

Trains between Katoomba and Lithgow in the Blue Mountains have been suspended in both directions, after a tree fell on overheard wiring at Blackheath.

Buses are transporting stranded passengers.

A tree fell across a rear train carriage at Medlow Bath last night shortly after 8pm (AEST).

An elderly man was taken to hospital with minor head injuries.

A tree also fell on a high-speed XPT train, but no one was injured.

A nursing home at Leura had its roof blown off last night.

Endeavour Energy estimates 24,000 homes are without power in the worst affected parts of the Blue Mountains, southern highlands and the south coast this morning.

"We're expecting to make good progress but it will probably take us until mid-morning to get a real handle to restore customers," spokeswoman Kate McCue told AAP.

The State Emergency Service has received 1000 calls for help since late last night, with most requests a result of fallen trees in the Blue Mountains, the southern highlands and the south coast.

Spokeswoman Becky Gollings said 200 volunteers were ready to help, but strong winds had made it too dangerous for them to be outside.

Power outages across the Blue Mountains have also caused traffic lights to black out.

Scattered branches have closed sections of the Great Western Highway, with the NSW Roads and Traffic Authority urging motorists to avoid the road and use the Bells Line of Road in the Blue Mountains instead.

Fallen trees have also closed the Illawarra Highway in both directions, and the Jamberoo Mountain Road between Jamberoo and Robertson.

The Bureau of Metrology is expecting damaging winds to return later on Wednesday, with peak gusts of 100km/h forecast for parts of the Hunter, south coast, Illawarra, Snowy Mountains, southern tablelands, central tablelands and the ACT.

-AAP