One in the trio, aged 61, was knocked unconscious by the strike. She was revived and later airlifted to Royal North Shore Hospital in a critical condition.

The women had taken cover under a tree at Corbett Gardens as a wild thunderstorm thrashed parts of the Illawarra and Southern Highlands, about 3pm Saturday. 

The Mercury understands the lightning struck an umbrella one of the sisters was holding. 

A passer-by attempted to revive the most seriously injured woman before NSW Ambulance paramedics and police arrived. 

The woman’s sisters were treated at Bowral hospital for burns and shock. 

Bowral was hit with a severe storm cell at about 3pm on Saturday. The storm moved in a south-easterly direction towards Kiama.

The Bureau of Meteorology reported golf-sized hailstones in the area. 

The unstable weather conditions stretched to Sydney, with severe thunderstorms hitting Sydney's northern suburbs on Saturday afternoon.

"There have been really strong steering winds pushing the storm in, quit deep easterly winds have stretch it out all the way to the coast in some form," Jordan Notara of the Bureau of Meteorology said.

"Although we haven't seen reports of large hailstones on Sydney's coastal fringe."