Corrective Services and police are investigating after a 19-year-old man died at Long Bay Correctional Complex in Sydney on Sunday.
Within the prison, he had been staying in the Metropolitan Special Programs Centre, which the inspector of custodial services recommended be closed after finding rusted furniture, ligature points, mouldy walls and evidence of vermin, according to its 2023-24 report.
Greens MP Sue Higginson urged the state Labor government to change its approach to incarceration.
"Whenever we talk about prisons and 'bad people' and sick people - people who have fallen on the wrong side of the tracks - we're talking about real human beings that have lives, have people who love them and deserve to have a future," she told AAP.
"But at the moment, we're dehumanising people and it is a reflection of our humanity.
"Since coming to power, the Minns government has run a populist law and order agenda."
More than 14,000 people were imprisoned in NSW as of March, the highest figure recorded, according to the state's Bureau of Crime Statistics and Research.
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.
Between November 2025 and March 2026, the number of inmates in NSW rose by 8.2 per cent, meaning the population has increased more in 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 NSW Bureau of Crime Statistics and Research executive director Jackie 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 told AAP.
While she acknowledged prisons could help incapacitate offenders and keep victims safe, this might 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.
Premier Chris Minns agreed the state's jails needed to be modernised and changed, but backed policy changes that had led to the growing remand population.
"Policy solutions to the large prison population are not to just open the gates," he said.
"We're serious about our law changes and we think they've saved lives."
Experts have noted incarceration can often lead people to commit more crimes and Ms Higginson said addressing domestic violence must include preventative measures such as education programs for young men.
"When you are putting people into facilities that are not designed to treat people ... you are always making them more criminal,'' she said.
"We need to make sure that we are providing them with the much-needed education services, support and drug substance rehabilitation programs."
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