Serial killer Ivan Milat is getting a break from Australia's toughest jail to undergo a medical procedure in a Sydney hospital.

Key points: Officers from extreme high security unit escorted Milat to a Sydney hospital

Officers from extreme high security unit escorted Milat to a Sydney hospital While in hospital he will have to wear restraints, either ankle cuffs or handcuffs

While in hospital he will have to wear restraints, either ankle cuffs or handcuffs Source close to Milat's family said he's lost 20kg and is being assessed for possible organ failure

Milat murdered seven backpackers and dumped their bodies in the Belanglo State Forest in NSW.

Officers from the extreme high-security unit at the Goulburn Correctional Centre supermaximum security prison escorted the convicted murderer to the Prince of Wales Hospital in the Sydney suburb of Randwick.

A source close to Ivan Milat's family told the ABC the 74-year-old has undertaken a series of hunger strikes in prison over the past decade in protest against his treatment by prison guards.

The ABC understands Milat is in the advanced stages of cancer after several lumps were found in his stomach and his throat.

It is unlikely he will return to his cell at Goulburn Correctional Centre. The ABC understands arrangements are being made to transfer him, possibly to another prison such as Long Bay jail hospital.

He has recently lost 20 kilograms.

In a statement, NSW Corrective Services said "extensive security and planning" was undertaken before the 74-year-old was transported to hospital.

It said Milat was searched before leaving the prison in Goulburn and "will be searched again on his return".

While in hospital Milat will have to wear restraints, it said.

"At least one form of restraint — handcuffs or ankle cuffs — remains on high-risk inmates during medical treatment, subject to medical requirements," a spokeswoman said.

The yard at the Goulburn Correctional Centre. ( Supplied: Corrective Services NSW )

Special hospital annex for treatment of prisoners

Milat was admitted to a special annex within the hospital, fitted out for the treatment of prisoners with reinforced block walls and steel-lined ceilings.

He was jailed in 1996 and is serving seven life sentences for the brutal murders of seven backpackers.

Their bodies were dumped in the Belanglo State Forest in southern NSW between 1989 and 1993.

A $47 million injection from the NSW State Government, announced in February, will see the complex broken into two separate wings.

At the time, a spokesperson from NSW Corrective Services told the ABC it is hoped the prison will one day act as a "halfway" between the supermaximum security prison and a regular prison.

According to the plan, eligible inmates will in future be rehabilitated there and eventually moved to lower security settings within the facility.