Two people struck by lightning in Sydney

A man finishing his swim at the beach was among two people struck by lightning within 10 minutes in Sydney

The man, aged in his late 20s, was struck about 5.24pm while leaving the water at Dee Why Beach, NSW Ambulance said on Monday.

After being given CPR by a friend and then paramedics, he was taken to Royal North Shore Hospital in serious condition.

Just 10 minutes earlier, a 71-year-old woman in Arcadia was struck in the chest by a lightning bolt.

Careflight said it understood the woman was standing underneath a tree in her Arcadia residence when the lighting struck it and her.

She was treated by passers-by and paramedics before being flown to Westmead Hospital in a critical condition

Police and firefighters helped douse the tree, which caught fire.

NSW Ambulance Inspector Carolyn Parish said the circumstances were "extraordinary" and credited the first responders.

"This is the first time I have ever come across two lightning strikes within 10 minutes - it's terrifying," she said in a statement.

"How you react to being hit by lightning depends on the circumstances and where you have been hit.

"Most people, however, will go into cardiac arrest so those first moments of CPR are vital."