Paul Douglas Peters had caused unimaginable terror with his actions, Judge Peter Zahra said.

Peters will be eligible for parole in August 2021 - a non-parole period of 10 years.

On August 3 last year, Peters, 50, wearing a bright, rainbow-coloured balaclava, walked into the $12 million Mosman home of 18-year-old Madeleine Pulver and tied a black metal box around her neck with a bike chain.

Attached was an extortion note claiming the box contained sophisticated plastic explosives and threatening Ms Pulver and her family with a "tragically avoidable explosion" if they failed to send "funds" to a supplied email address.

It took 10 hours for police to be certain the device was a fake, by which time Ms Pulver had been traumatised.

Over the coming days, state and federal police launched a major investigation that ultimately led them to Louisville, Kentucky, where Peters was arrested on August 15.

After initially denying any involvement with the crime, Peters pleaded guilty to aggravated break and enter and detaining for advantage in March this year, much to the relief of his teenage victim and her parents, Bill and Belinda Pulver.

In sentencing Peters on Tuesday, Judge Zahra rejected the claim that Peters was affected by a disturbed state of mind and had ultimately wanted to be caught.

Instead, he found Peters was seeking financial gain through a carefully planned extortion attempt and that he was aware of the terror he was inflicting.

"I'm not prepared to accept that the offending was the product of being in a psychotic state," Judge Zahra said.

"I'm not prepared to accept that he was taking on the persona of a character from his book [that he was writing]."

"He would have appreciated the enormity of what he was doing and the impact on the victim.

"His mental condition did little to reduce the moral culpability - he would have understood that his victim was in fear for her life."

The standard non-parole period for the offences of break and enter and detaining for advantage is five years, but Judge Zahra said the nature of the crime put it in the most serious category, requiring a much longer jail sentence.

"The sentence calls for a strong element of deterrence, both for the specific offender and this offender as an individual.

"The offender placed the victim in fear that she was going to die in order to extort money from her family.

"The victim was vulnerable, being entitled to the sanctuary of her own home. She was on her own studying for her trial HSC exams. The terror she experienced can only be described as unimaginable."

Judge Zahra said Peters's expression of remorse had been "guarded and qualified" and that he could not find he was truly sorry for what he had done.

Peters stood, expressionless, as his sentence was handed down.

Madeleine Pulver sat between her parents who put an arm around her as the judge read his decision.

A few minutes later, with the enormity of the event seemingly seeping in, Bill Pulver broke into tears, and was hugged and comforted by his daughter who was also crying.

Sydney's District Court heard that the mental and emotional impact on the young woman had been severe, including a number of ongoing psychological conditions which cannot be disclosed fully for privacy reasons.

Outside court, Madeleine Pulver said she and her family could now look to a future "without Paul Peters's name being linked to mine.".

"I'm pleased with today's outcome. I realise it's going to take quite some time to come to terms with what happened but today was important because now the legal process is over.

"For me it was never about the sentencing, but to know that he will not re-offend and it was good to hear the judge acknowledge the trauma he has put my family and me through," she said.

"It's been a surprise to me that this year has been much harder than last year but I’m lucky enough to have family and friends and we are all making great progress."

"One of the reasons I wanted to come today was out of respect for all those involved, particularly the NSW police force, the DPP and all the support we have received.

"Looking forward I’m heading to Sydney Uni next year and I’m really looking forward to it."

Bill Pulver said his daughter was a "very, very special young lady who has handled herself with incredible poise and dignity throughout this trial".

Detective Superintendant Luke Moore, who headed the police investigation, said the case was "one of the strangest cases I will ever work on".

"We've never really seem anything like this before. I think it's a fair sentence given the circumstances of the crime.

"I'd like to pay tribute to Madeleine Pulver and her family. It was very difficult for her to be here today and she showed a lot of courage."

Obsessed with book

It emerged during earlier sentencing hearings that Peters had become obsessed with a science fiction book he was writing - a dark dystopia that included among its pages the kidnapping of a young girl by a black-hearted protagonist.

During three hearings, Peters's lawyers argued that, affected by bi-polar disorder, alcoholism and depression brought on by the loss of his job and his marriage, he had committed the crime under the delusion that he was the central character in his novel.

But the prosecution painted a very different picture.

Crown Prosecutor Margaret Cunneen, SC, said Peters had been angry at losing his wealth and status and had decided "get it all back in one go".

She said the 52-year-old had come to Sydney from his home in the US intending to extort money from the beneficiary of a large trust fund - the James M Cox trust.

He had abandoned this target when he stumbled across a prominent Sydney businessman who lived on the same battle-axe block as the Pulvers.

Bizarrely, according to the Crown, it was a simple mistake which led Peters to the Pulvers's door — he had mistaken the Pulvers's house for that of his intended target.

Having pleaded guilty early on, Peters received a 25 per cent discount on his sentence.