The man responsible for the death of a gay American mathematician who had "everything to live for" has been sentenced to nine years in jail.
Having already served part of his sentence, Scott Phillip White will be eligible for release in 2026 for punching Scott Johnson, leading him to fall to his death in the late 1980s in Sydney.
"Not much is known of the death beyond a punch on a cliff, a fall from a cliff and decades of pain and grief that followed," Justice Robert Beech-Jones said during sentencing in the Supreme Court on Thursday.
On December 10, 1988, White met Dr Johnson at a hotel and the pair went for a walk around North Head which was known at the time to be a gay beat.
White, then 18 years old, threw a punch at Dr Johnson in the heat of an argument causing the 27-year-old to stagger backwards and fall to his death over the cliff.
The American was close to receiving his doctorate from the Australian National University, which he has since been awarded posthumously.
"Dr Johnson was an American citizen ... he had everything to live for," Justice Beech-Jones said.
"The offender left (him) to die."
White was jailed for 12 years and seven months in 2022 after making a startling guilty plea to murdering Dr Johnson.
The accused killer successfully overturned his guilty plea and conviction in November before pleading guilty to the lesser charge of manslaughter in February.
The 52-year-old had long denied murdering Dr Johnson, only to say "guilty, I am guilty" in a surprise plea during a pre-trial hearing in Sydney in January 2022.
He later told his confused lawyers at the time "I didn't do it but I'm saying I'm doing it ... it's the only way, she's going to come after me" - suggesting his former wife would attack him.
White, who now has early onset dementia due to excessive alcohol abuse, was described as a Manly "'street kid' at the time of the killing.
"The offender was clearly a damaged albeit physically powerful young man," Justice Beech-Jones said.
"However he was not broken as he is now."
Originally thought of as a suicide, NSW Police eventually opened an investigation into what they thought was a gay hate crime in 2012.
In her now overturned judgment on the murder conviction, Justice Helen Wilson found there was not enough evidence to show the attack was motivated by Dr Johnson's sexuality.
Justice Beech-Jones said he could not be satisfied beyond reasonable doubt that the crime was a "gay hate crime".
"Answers to numerous other questions about how he died, why he died and what happened ... some of those answers will never be provided," Justice Beech-Jones said.