Drivers were trapped in cars, trains were halted and tourists were struck by lightning as severe storms rolled across NSW, bringing destructive winds and large hail on Monday.

Key points: Hail has hit parts of Sydney's west and fallen trees have suspended some train services

Hail has hit parts of Sydney's west and fallen trees have suspended some train services Two people were struck by lightning in the Blue Mountains

Two people were struck by lightning in the Blue Mountains ACT's emergency services agency received over 1,750 requests for help

A large swathe of the state spanning the Hunter, the Riverina and the South Coast was hit by wild weather.

The low pressure system initially brought welcome rain to the fires still smouldering in the Snowy Mountains and South Coast before heading towards Sydney.

Just after 2.00pm a pair of tourists were struck by lightning near Echo Point at Katoomba in the Blue Mountains National Park.

A 16-year-old boy was climbing a metal staircase and a 34-year-old was holding a metal railing when lightning hit.

Both suffered electrical shock injuries and the teenager has entry and exit wounds to his arms and feet and burn marks on his torso.

Paramedics treat a 16-year-old boy who suffered minor burn marks. ( ABC News )

Both are in a stable condition in Nepean Hospital.

Later in Miranda in Sydney's south, three people were trapped in their cars for around 30 minutes after large trees fell on President Avenue.

This woman was among three drivers trapped in their cars after trees fell in Miranda. ( ABC News )

Two drivers were treated at the scene and the third was taken to Sutherland Hospital in a stable condition.

At about 5:00pm trains on the T4 line were suspended between Cronulla and Sutherland after a tree fell on the tracks, leaving some passengers trapped on a train for more than an hour.

Three people were also briefly trapped in a bus when a tree fell in Bonnet Bay in Sydney's south.

In Sydney's south-west, the Bureau of Meteorology (BOM) said Campbelltown was pelted by "giant" hailstones.

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Nearby Oran Park had 4.5 centimetres of hail while Camden, Harrington Park, Elderslie and Kirrawee were also impacted.

In Bringelly, winds ripped part of the roof off at a small shopping complex, with debris strewn across the road.

Winds blew large chunks of roofing off in just 30 seconds. ( ABC News )

The nation's capital was smashed by hail earlier in the day— the National Archives of Australia had to close for the safety of visitors and staff, with one employee saying the hail sounded "like gunfire".

The ACT Emergency Services Agency said it received over 1,750 requests for help.

Hail piled up outside Parliament House in Canberra. ( Twitter: @otiose94 )

But in St George's Basin, on the edge of the Currowan fireground near Nowra, Spike Dale was celebrating the dramatic change in conditions.

"Lately the view across St George's Basin is smoke. Today it's glorious rain," he said.

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It has been a wet few days for NSW with some areas receiving big falls — 243 millimetres in Tumbulgum, 250mm in Clothiers Creek and 212 mm in Bilambil Heights over a 24-hour period.

Farmers in some drought-stricken areas have been overjoyed with the rain which has filled dams, and instantly made grass greener.