The device was "very elaborate, very sophisticated" and was placed near the 18-year-old woman, New South Wales state Police Assistant Commissioner Mark Murdoch said. But he refused to confirm reports the device was tethered to the woman's body.
Police are looking for a person who they believe placed the device in the suburban Sydney home, he said. "We want to get our hands on who's done this," Murdoch told reporters.
During the efforts to free the woman, Murdoch said no one claiming to have left the device contacted police.
The teen was doing well after being freed and was reunited with her parents, who had been kept out of the house by police during the ordeal for their own safety. The young woman was being taken to a hospital for an examination, police said.
"She's good - she's been kept in a very uncomfortable position," Murdoch said. "She has been and will be uncomfortable for a little while to come."
Murdoch said it was "far too early to say" whether the device was part of an extortion attempt and refused to comment on a report that a note was left with the device. He said police have no ideas as to a motive.
"The family are at a loss to explain this," he said. "You would hardly think that someone would go to this much trouble if there wasn't a motive behind it."
Police said they did not consider it a case of "self-harm" and that the woman tried vigilantly to help police since she called them to her home about 2:30 p.m. in the wealthy suburb of Mosman.
The device was still intact upon her release, Murdoch said.
"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. "The examination of that device will continue into the early morning and possibly longer."
Murdoch said the state police consulted with Australia's federal police agency and the British military during the ordeal.
"We have left absolutely no stone unturned to make sure that we met our objective - and that objective always was the safety of the young lady," he said.
Investigators planned to comb the house for clues throughout the night.
Murdoch said the case was one of the most bizarre he'd seen in his career.
"I've been doing this job a long time and this is the first time I've come across anything like this," he said.
The woman has not been identified as Australian law enforcement very rarely release names of victims or suspects in criminal investigations.