Four people have been taken to hospital after being struck by lightning in Sydney’s inner city during a sudden storm.

The Ambulance service said the group – a teenage boy, a woman in her 20s, and a man and woman both in their 30s – were sheltering under a tree at the botranic garden when they were struck by lightning just before 1pm.

All four were knocked unconscious and treated for burns and cardiac problems by paramedics.

They were rushed to hospital in a stable condition.

A sudden storm swept along the NSW east coast leaving an estimated 19,500 homes without power in northern Sydney and the Hunter region.

Flights were also delayed at Sydney Airport.

Nearly 50mm fell in an hour at Kings Langley, in the city’s northwest, while Belrose received 34mm in two hours.

Near-stationary storms also caused downpours over Blacksmiths in Newcastle with 65mm falling in less than six hours.

About 210,000 lightning strikes were detected within 200km of Newcastle between 5am and 11am on Monday by Weatherzone’s Total Lightning Network.

A separate severe thunderstorm warning is also in place for northern NSW ranging from Taree and Kempsey along the coast, to Coonabarabran and Inverell inland.

The Bureau of Meteorology issued a severe weather warning for heavy rainfall, damaging winds and large hailstones for metropolitan Sydney, parts of the Mid North Coast, Hunter, Illawarra, Central Tablelands, North West Slopes and Plains, Northern Tablelands and Central West Slopes and Plains Forecast Districts.

Tweet from @BOM_NSW

The storm is expected to linger into the afternoon, with flash flooding warnings for Taree, Newcastle, Gosford, Sydney, Parramatta and Armidale.

The NSW State Emergency Service has received more than 90 calls for help across the state, with 38 in the Sydney metropolitan area.

An automatic flash flooding alarm was triggered on Bexley Road in Earlwood.

-with AAP