A 19-year-old man died at the Long Bay Correctional Complex in southern Sydney on Sunday, a spokeswoman for Corrective Services NSW said on Thursday morning.
"Staff commenced a medical response, but he was pronounced deceased by paramedics on Sunday," the spokeswoman said.
Corrective Services and police are investigating and the death will be referred to the coroner.
More than 14,000 people were imprisoned in NSW as of March, the highest figure ever recorded, according to the state's Bureau of Crime Statistics and Research (BOCSAR).
The surge has been driven by a record high number of people being held on remand, which is those charged but not convicted who are often awaiting trial, rather than an increase in crime rates.
This has not been caused by a change in crime rates, but by police laying more charges - particularly over domestic violence offences.
The uptick reflected similar charging patterns in the aftermath of the 2014 Lindt Cafe siege, where a man on bail held 18 people hostage in the Sydney CBD.
"The Lindt Cafe siege had particular issues around that offender and questions about his release on bail," NSW BOCSAR executive director Jackie Fitzgerald told AAP.
"That was a bit of a wake-up call and a shake-up for the judiciary, and we did see a sharp change in bail decisions after that at a similar magnitude."
But unlike aftermath of the hostage situation, bail refusal rates have not changed and instead there has been an influx of people entering the criminal justice system.
Between November 2025 and March 2026, the number of inmates in NSW rose by 8.2 per cent, meaning the population has increased more over four months than in the previous four years.
Almost half of all people in custody are on remand, with 41 per cent of this increase attributed to those charged with domestic and family violence.
The figures reflect NSW Police's increased focus on domestic violence, which Ms Fitzgerald said was a welcome development.
But she said more effort must be invested in approaches outside the justice system.
"We have to look at strategies to keep victims safe and prevent these offences from occurring ... so we don't have a situation where the expensive justice system is our go-to strategy for responding to domestic violence," Ms Fitzgerald said.
While she acknowledged prisons could help incapacitate offenders and keep victims safe, this may not be viable in the long term.
"That could mean bringing forward the building of a new prison and that's a very costly exercise," Ms Fitzgerald said.
However, just maintaining the current facilities is proving challenging with the state inspector of prisons calling for the Metropolitan Special Programs Centre, where the 19-year-old died on Sunday, to be permanently closed.
Cells at the were found to have ligature points, mouldy walls and evidence of vermin in a review conducted by the inspector in 2023 and 2024.
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