Mustafa Kayirici was to spend at least another forty years in jail but was diagnosed with terminal cancer and given a life expectancy of just months.

The 31-year-old died in the secure prison annex at Prince of Wales Hospital on Tuesday, Corrective Services NSW said in a statement.

He was found unresponsive in his cell and pronounced dead at 10.25am.

He was serving multiple sentences following numerous charges for rape including several counts of aggravated sexual assault on a child under 16 in 2016.

While the state was in a hard lockdown in August 2021, Kayirici pleaded with the NSW Supreme Court to allow visits from his younger sister.

These were initially deemed too risky by the Corrective Services Commissioner, who noted his past offending and his "volatile behaviour" while in custody, Justice Geoffrey Bellew said.

But after further review of his medical records the commissioner indicated these visits could go ahead in future "on compassionate grounds," but due to the rapid spread of COVID-19 must be conducted via audio-visual link in the meantime.

Kayirici, representing himself via audio-visual link, told the judge his cancer had spread to his lymph nodes, lungs and brain, and he had been in Long Bay Prison's hospital for 16 months.

"Time is not on my side Your Honour."

He said his main aim was to allow his sister to be included with other family member visits.

"It's not fair for her to feel left out."

Justice Bellew noted Kayirici's deteriorating health and acute medical needs, recommending his application be dealt with expeditiously.

Kayirici's death, like all in custody, will become the subject of a coronial inquest.

Australian Associated Press