© Facebook/AAPMadeleine Pulver endured 10 hours attached to a possible bomb before police we able to free her
Police say no explosives have been found at a Sydney North Shore home where a young woman was trapped for 10 hours with a suspected bomb strapped around her neck.

Eighteen-year-old Madeleine Pulver was reunited with her parents shortly after midnight after the ordeal at the Mosman house.

Police called in British military experts for help during the stand-off, which was reportedly part of an extortion attempt.

The drama began when Ms Pulver called police to the Burrawong Avenue house at about 2:30pm on Wednesday.

She was inside alone with the box-shaped device strapped to her neck when officers arrived.

Earlier reports suggested the device was placed on her neck by a man in a balaclava. There were also reports the man left a note in what is suspected to be an extortion attempt.

Assistant Commissioner Mark Murdoch says the offender had left instructions which contained a "precise ... serious threat".

Ms Pulver is the daughter of William and Belinda Pulver, who are reportedly one of Sydney's wealthiest couples.

Earlier this morning Commissioner Murdoch said the device had been removed intact.

"It is a very elaborate, very sophisticated device... the manner in which it was located in proximity to the young lady was such that it has taken us the better part of 10 hours to secure her release," he said.

"Certainly anything that has taken us and our bomb techs, who are amongst the best in the business anywhere in the world, taken them 10 hours to come to grips with, and make sure that the young lady is safe, is a big task.

"The device and the scene, as best we could, were examined for explosives on a number of occasions."

However police later confirmed that no explosives had been found.

The investigation is being led by the Robbery and Serious Crime Squad, which deals with extortion.

Detectives are investigating whether the Pulver family has been threatened in the past but are refusing to say whether the bomb scare is linked to a recent series of extortion attempts threatening harm to the children of Sydney families.

Streets in the area were cordoned off during the incident and police have set up a roadblock in the street, which is also home to horse trainer Gai Waterhouse.

'Family at a loss'

Assistant Commissioner Murdoch said it was too early to speculate on a motive for the crime.

"Certainly the family are at a loss to explain this, but you wouldn't expect someone would go to this much trouble if there wasn't a motive behind it," he said.

Police have confirmed that they have not been in contact with the person who left the device with Ms Pulver.

During the ordeal she was unable to speak to her parents, who were kept from the house due to safety concerns.

But Assistant Commissioner Murdoch says she was able to remain calm while officers worked to free her.

"She's been kept in a very uncomfortable position for in excess of 10 hours, she has been and will be uncomfortable for a little time to come, but she's in good hands," he said.

"She's with mum and dad, who are the most important people to be with at this point in time, and she'll be cared for and she'll be all right.

"Also the family have endured something no-one needs to endure, and as a parent certainly something I would never want to contemplate.

"But they have held up remarkably well, they are obviously immensely relieved their daughter is now safe, and are spending time with her comforting her now."

Assistant Commissioner Murdoch said it was the first time he had seen anything like this.

"This is an unusual incident for New South Wales and indeed Australia. I'm not aware of anything like this happening in the country before, and I think that is what's made it so unusual," he said.

He also praised the efforts of the bomb disposal squad, who were involved in the rescue.

"The important thing is that the young lady is safe," he said.

"It is a testament to the skills of our bomb technicians, our New South Wales police force bomb technicians, who have worked tirelessly to secure the young lady's release."

Police are calling for witnesses and anyone who might have information about the situation to call CrimeStoppers on 1300 333 000.