The injuries suffered by the two men and two women occurred during one of several severe thunderstorms passing over Sydney on Monday afternoon.
The quartet, aged from teens to their 30s, had been sitting under a tree in the gardens when it was hit by lightning at about 12.45pm.
All four sustained burn injuries and were taken to hospital in a stable condition, NSW Ambulance said.
Sydneysiders have been warned the thunderstorms passing over the city are likely to produce heavy rainfall in some areas, leading to possible flash flooding.
Particular areas of concern are the eastern and northern suburbs and areas north of Penrith and Colo Heights in the city's outer west.
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 storms are likely to produce damaging winds, large hailstones and heavy rainfall that may lead to flash flooding during Monday afternoon.
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