Aymen Terkmani, who brutally killed teenage boy, stabbed to death ...

A Sydney killer serving a sentence of at least 33 years for the “horrific” sexual assault and murder of a 16-year-old boy has died in prison.

Aymen Terkmani, 31, was stabbed to death at Lithgow Correctional Centre in Marrangaroo just before 2.45pm on Wednesday, NSW Police said.

Paramedics treated Terkmani for a stab wound to his chest before he was airlifted to hospital.

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However he died on the way.

“The facility was locked down and a crime scene has been established,” NSW Police said in a statement.

Terkmani, was found guilty in 2017 of sexually assaulting and murdering Mahmoud Hrouk in May 2015 at an unoccupied Fairfield East home, The West previously reported.

In the NSW Supreme Court, Justice Lucy McCallum sentenced him to a maximum of 45 years in prison with a non-parole period of 33 years.

“The offender subjected the victim to the most brutal and horrific attack, inflicting injuries too numerous to list and too gruesome to describe,” McCallum said.

Terkmani declined to give evidence at his trial and never revealed any kind of motive.

Lithgow Correctional Centre is a maximum-security facility for men, and is about 152km west of Sydney in the Blue Mountains.

It is home to mass murderers, organised crime bosses and serial rapists.

The late serial killer Ivan Milat, who murdered backpackers in Belanglo State Forest, once served time at Lithgow.

His great nephew Matthew Milat is now at the prison, after killing a friend with an axe in the same forest in 2010.

Brothers 4 Life gang founder Bassam Hamzy was also at Lithgow, but is now in Goulburn’s Supermax.

“Granny Killer” John Wayne Glover died at the prison in 2005.