Police are trying to defuse a bomb strapped to an 18-year-old girl in a multimillion-dollar mansion at Burrawong Avenue in Sydney, Australia.

The situation reportedly began to unfold when the teenage girl placed a call to the police. It was unclear how the device came to be strapped to the girl, but it is understood she did not place it there herself, The Daily Telegraph reported.

According to a senior police officer, the device is an unusual "collar bomb," which has never been seen before in Australia; an exact description has yet to be officially given.

"We don't know what we are dealing with. We are working very hard to find out exactly what it is, and equally important, what it isn't," said Assistant Police Commissioner Mark Murdoch. He did not say whether the girl was a victim of an extortion attempt.

It has been speculated that an intruder wearing a balaclava entered the house, attached the explosives to the girl and left a ransom note attached to her neck, but bomb experts have not been able to read its contents, and are still examining the device.

According to Murdoch, the girl -- who is part of one of Sydney's wealthiest families -- has not moved from a room at the front of the house in more than six hours.

"The young lady remains in the house with the device, and specialist police are trying to verify what we are dealing with," he said. "Until we know what we are dealing with, we will treat it with the utmost seriousness. We are still treating the suspicious package as live," Murdoch said about 7 p.m. "I can't confirm whether it is strapped to the woman involved, but she is still in the vicinity of the device."

When asked if the girl could move away from the bomb, Murdoch said, "No, she can't get away from it."

Senior police officials believe it is an extortion attempt and the initial details of the situation were being conveyed to police via the girl's father.

Police have sectioned off the house and a command post has been set up on the corner of Burrawong Ave and David Street. The street is home to racing identity Gai Waterhouse, ex-Wallaby Phil Kearns and John Eales.

The young woman's parents, who have not been identified, were outside the house comforting each other.