Police have decided to formally caution rather than charge two teenage boys who allegedly threw trolleys off the roof of a shopping centre in Sydney's south, injuring a woman.

The pair went to St George police station this morning after CCTV footage of a group wanted in connection with the incident was released.

A 50-year-old woman was sitting in a stationary car at the Hurstville centre on Tuesday when a trolley smashed the windscreen.

Moments before this, the driver had stopped the car to investigate after hearing a loud bang.

"The man got out of his car, when he saw a shopping trolley fall from the roof of a shopping centre and land on a bus nearby," a police statement said.

"A short time later, a second trolley was thrown, falling onto the man's car and smashing the windscreen where the woman was still sitting.

"Emergency services were called and ambulance paramedics treated the 50-year-old woman at the scene for minor injuries."

The nearby bus was also damaged.

Detective Inspector Paul Simpkins said the woman could have been killed.

"This act has resulted in damage to two vehicles and a woman being injured," he said.

"The incident had the potential to cause serious injury or even death.

"I'd appeal for anyone with information that may assist detectives to come forward."