An 18-year-old Sydney schoolgirl strapped to a suspected bomb for 10 hours is safe after a horrifying ordeal that has left her wealthy family at a loss as to why she was targeted.

A manhunt is underway for the attacker who was wearing a balaclava when the sophisticated and complex device was attached to the young woman's body.

The family of 18-year-old Madeleine Pulver endured "something no one needs to endure", after she was attached to the device during an attack in her Mosman home on Wednesday, police said.

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Police are now hunting for whoever carried out the bizarre attack, which left police puzzling for hours over how to defuse the "elaborate, sophisticated" device.

Police say the family, reportedly one of Sydney's wealthiest, had no idea why they were targeted and Assistant Commissioner Mark Murdoch told reporters it was "far too early" to establish a motive.

Madeleine's father, William Pulver, is reportedly chief executive of Appen Butler Hill, a linguistic solutions company.

Mr Murdoch refused to confirm whether a ransom note had been left at the scene, saying only that police had had no contact with who ever carried out the attack.

The incident began about 2.30pm (AEST) on Wednesday when Madeleine alerted police, telling them she had been attacked and was strapped to a bomb.

They found her alone in her parents' home on Burrawong Avenue in wealthy Mosman, with the device reportedly collared around her neck.

A terrifying 10-hour ordeal ensued, during which Madeleine was not allowed to speak to her parents for operational reasons.

Instead, she was comforted by two police negotiators, who kept her talking, warm and fed, as two bomb disposal experts worked to defuse the device.

Madeleine managed very well considering the circumstances, and did a good job of keeping her emotions in check, Mr Murdoch said.

Outside, the area swarmed with police, bomb squad technicians, rescue squad officers, firies, State Emergency Service (SES) volunteers and paramedics.

Nearby mansions on the street, whose residents include horse trainer Gai Waterhouse, were evacuated and police cordons were put in place.

The devise, described by Mr Murdoch as "a hard nut to crack", took specialist bomb disposal technicians 10 hours to defuse.

During that time, advice was sought from several agencies, including the British military and the Australian Federal Police bomb centre.

It was eventually removed from Madeleine around midnight on Wednesday, still intact, and she was immediately reunited with her parents.

She has since been taken to Royal North Shore Hospital for treatment.

Mr Murdoch said police still did not know if the device was explosive.

But he said Madeleine's safe release was a "testament to the skills of our NSW bomb technicians who have worked tirelessly".

The focus is now on finding the attacker, and police say Madeleine gave them a lot of information about.

"We want to get our hands on who has done this and pretty smartly," Mr Murdoch said.

"The family have endured something no one needs to endure ... but they have held up remarkably well," he said.

Mr Murdoch said police had not been able to give out information during the incident, because they did not know who was listening or watching the news broadcasts.

"Her safety was our prime objective ... and the motivator for that," he said.

He refused to reveal further details on Madeleine's ordeal, saying these would come out "in the fullness of time".

The investigation will continue to be led by the State Crime Command's robbery and serious crime squad, which deals with extortion.