An Australian teenager had a “collar bomb” strapped to her neck after an intruder broke into her home in Sydney, it has been reported.

The 18-year-old, from one of Australia’s wealthiest families, spent several hours with the bomb strapped to her as police tried to defuse it.

A ransom note was apparently attached to the collar bomb, which is still thought to be live. Explosives experts have been called in.

The girl, from expensive Sydney suburb of Mosman, was said to be “petrified”. As her parents stood outside in tears, police sealed off surrounding streets and evacuated nearby homes.

Assistant Commissioner Mark Murdoch, who arrive on the scene after the teenager put in a frantic call to emergency services, said it was a "highly delicate" operation.

"There's a device in the house in the vicinity of the young woman. It is still being examined as we speak.

"The young lady is doing her best to assist the police to find out exactly what's gone on.

"I can't confirm whether it is strapped to the woman involved but she is still in the vicinity of the device."

When asked whether the girl could move away from the bomb, the police officer added: "No, she can't get away from it."

Roger White, a reporter with 2UE Radio in Sydney, said police were appealing for local residents to come forward if they had seen anyone in the area who should not have been there.

"We understand she had this device placed around her neck, with a ransom note attached to it, by someone who burst into her home and then took off," he said.