A failed businessman who strapped a fake collar bomb to a Sydney teenager's neck pleaded guilty in a city court today.

Paul Peters broke into the Pulver family's home in Mosman in August last year and strapped the fake bomb around 18-year-old Madeleine Pulver's neck before fleeing the house, leaving behind an extortion note.

Appearing in Sydney's Central Local Court via video-link this morning, Peters entered guilty pleas on charges of aggravated break and enter and detaining the teenager for advantage.

Peters was silent throughout the hearing and showed no emotion.

The shock plea brought an end to a legal saga which made headlines around the world.

Peters fled to the USA after the extortion attempt and was arrested at his ex-wife's home in the US state of Kentucky a fortnight later.

He chose not to fight his extradition to the US but always maintained he would plead not guilty once back in Australia.

Madeleine Pulver was in court for today's hearing, along with members of her family.

Speaking outside court her father Bill Pulver said the plea was a relief, and said the family was grateful for the support shown to it by "thousands" of well-wishers.

"We are incredibly pleased with today's outcome. It is great comfort knowing Maddie won't have to endure the stress and anxiety of reliving the events of that terrible night," he said.

"Today's guilty plea brings closure to a crime that remains a mystery and as random to us in our mind as it did back on August 3."

A masked Peters ambushed Ms Pulver as she was at home alone studying for her HSC.

Police were so baffled by the fake bomb that they called British military experts for advice, and it took 10 hours for the device to be declared safe.

Peters, a 52-year-old father of three and former executive of the collapsed Allco finance company, will remain in custody until he is sentenced later this month.

But while Peters now admits he attached the fake bomb to Ms Pulver, he is remaining silent about just why he did it.

Criminologist Wayne Petherick from Bond University says it is a unique crime.

"This case was fairly unique in Australia in that it is probably about the first collar bomb type situation we've had," he said.

"Those kinds of things are usually reserved for some of the South American countries and some places in Europe, so just from that fact alone it was very unique in terms of Australian crime."

Ms Pulver has been widely praised for her bravery throughout the ordeal.

The policewoman who helped comfort her, Constable Karen Lowden, was another brave figure.

"She had her moments of great poise. She was very calm. She was definitely quite stressed as well, but level headed. (She) dealt with it perfectly," Constable Lowden said.

"Being in the police you come to deal with the unexpected. I was not exactly clear on what we were going into but I did know that there was a girl that needed help and my main concern was getting to her."