Australian police are attempting to assist an 18-year-old girl who has allegedly had an explosive device strapped to her neck.

A ransom note was found with the device which was reportedly attached to the girl when a man wearing a balaclava entered a house in the upmarket Mosman area of Sydney.

The victim is reportedly a member of one of Australia' wealthiest families.

Police and an army bomb disposal team have surrounded the house on Burrawong Avenue and are treating the device as live.

A bomb disposal team has entered the house to offer support to the teenager and examine the device. “The process demands a high level of skill and must be meticulous,” a police spokesman said.

He would not confirm if the device had been strapped to the girl by someone she did not know but said the incident was “not being treated as self-harm”.

The incident began when the girl, who was home alone when the man entered the house, phoned police about 2.30pm (5.30am Irish time).

"The young lady at this particular point in time is fine and doing her best to assist police,” assistant police commissioner Mark Murdoch told Radio Australia. “I'm not confirming anything at this particular point in time, we just need to play our cards pretty close to our chest.”

Several nearby streets were closed to traffic, and medical and fire crews were on standby.