The 50-year-old was pronounced dead at the Prince of Wales Hospital secure prison annex at 9.25am on October 24.

The Mercury understands he had been diagnosed with terminal cancer and had spent some time separated from the general prison population.

In February 2018 Fesus was found guilty of having murdered his teenage bride Jodie Fesus 20 years earlier at their Shellharbour home.

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Police found the 18-year-old's partially uncovered body at Seven Mile Beach near Gerroa, about six weeks after she disappeared.

Fesus was charged in 2013 after he confessed to an undercover police officer.

But he later retracted his confession, attributing it to pressure exerted on him during the covert police operation.

Despite his continued denial of the crime, sentencing Justice Peter Johnson said he was satisfied Fesus strangled or choked his wife to death during an argument.

She was unhappy in the relationship and wanted to leave Fesus, who had "acted on his attraction towards other women, both before and after Jodie's death".

Before her body was found, Fesus made a "significant slip" when later filling out a pension application form by indicating his wife was dead and not missing.

Justice Johnson found Fesus had not shown any remorse.

"The offender demonstrated a capacity to turn his emotions on and off so as to give an appearance of grief when he thought it suited him," Justice Johnson said.

"Having attacked his wife in anger, the offender then acted in a cool and calculated way.

"The speed with which the offender reported the discovery of his wife's body to the Department of Social Security is in itself sign."

Fesus was jailed for 22 years, with a non-parole period of 16 years and six months.

Jodie Fesus' relatives danced in the streets of Sydney when he was jailed, but the children she shared with Fesus, Kimberley and Dylan, remained supportive of their father. The sentencing justice noted Fesus had lied to the children and convinced them he did not murder their mother.

"Kimberley and Dylan were raised by the offender and apparently do not accept that he killed their mother," he said at the time.

Relatives and friends of Fesus have paid tribute to him on his public Facebook page.

Wrote one: "The fight is over you are now pain free so sad you didn't get to come home to your babies and grand babies."

Others called him "a top bloke" and "the most loveable, kind-hearted man".

Another said he had "battled a long and lonely road of cancer".

A spokeswoman for Corrective Services NSW said Fesus' death was being investigated.

"All deaths in custody are subject to a coronial inquest," she said.

The Mercury has approached Kimberley and Dylan for comment.