Serial killer Ivan Milat has been transferred from the medical wing of Long Bay Jail to Sydney's Prince of Wales Hospital after his health took a turn for the worse.

The 74-year-old was diagnosed with terminal oesophagus and stomach cancer earlier this year.

Sources close to Milat said he had been "in agony" in recent weeks and believed he was "only days away from his death bed".

The ABC has been told the killer was transferred to Prince of Wales Hospital because the Long Bay facility could not provide adequate pain relief.

It is understood a few of Milat's siblings had visited his hospital bed when he was first diagnosed with terminal cancer, but he has not had visitors recently.

The NSW Department of Corrective Services confirmed Milat was transferred from the jail, in Sydney's south, to the hospital last Friday under high security.

In May, Milat spent two weeks at the Prince of Wales Hospital in Randwick receiving treatment.

He was admitted to a special annex within the hospital — with reinforced block walls and steel-lined ceilings — that is used to treat prisoners,

Ivan Milat is wheeled out of the Prince of Wales Hospital in May. ( ABC News )

Milat is serving seven life sentences for the murder of seven backpackers.

The bodies of the backpackers, aged 19 to 22, were discovered buried in the Belanglo State Forest, south of Sydney, between 1992 and 1993.

Milat was convicted in 1996 and was held in a high-security unit at Goulburn Supermax — home to Australia's worst criminals — until this May when he was transferred to Long Bay.

During his time in solitary confinement at Goulburn, he went on several hunger strikes and sometimes swallowed sharp objects if guards did not meet his demands.

Known as the Backpacker Killer, Milat has never shown remorse for murdering seven travellers.

NSW Corrections Commissioner Peter Severin said he hoped Milat would speak to police about other crimes he may have committed before he dies.