The Coroners Court of Queensland will have to consider the impact the sudden death of double murderer Garry Dubois may have on an inquest into the Whiskey Au Go Go arson attack, a spokesperson says.
Mr Dubois was found dead in his cell at the Maryborough Correctional Centre about 4.30am on Sunday, Queensland Corrective Services said.
Efforts were made to resuscitate the 74-year-old but he was declared dead at 5.20am, with QCS assisting Queensland Police to prepare a report for the coroner.
Dubois died before he could give evidence at an inquest into the 1973 arson attack on the Whiskey Au Go Go nightclub in Brisbane's Fortitude Valley, which killed 15 people.
He was not scheduled to give evidence during the two weeks of sittings starting on June 14, but further sittings are scheduled for later in the year.
A Coroners Court spokesperson said the impact of Dubois' death on the inquest "will need to be considered" but it would not cause an interruption to the start of the inquest.
Dubois' death in custody will be investigated by the Queensland Police Service's Corrective Services Investigation Unit and an inquest will be held.
Dubois and Vincent O'Dempsey had their appeal dismissed in 2018 after being found guilty two years earlier of killing Vicki McCulkin, 13, her 11-year-old sister Leanne and their mother Barbara.
The pair were jailed for life for killing the McCulkins in bushland near Warwick on the night of January 16, 1974, after abducting them from their Brisbane home.
Dubois was found guilty of raping and murdering Vicki and Leanne. He was convicted of the lesser charge of manslaughter against Barbara.
The trial heard he may have been motivated to kill Mrs McCulkin over fears she would try to implicate O'Dempsey in the Whiskey Au Go Go firebombing.
Dubois and O'Dempsey were expected to be witnesses at a new inquest launched into the fire, with pre-inquest hearings only getting under way in late April.
It's the second major blow for the new inquest after key witness James David Finch, died on the eve of pre-inquest hearings just over a month ago.
The Londoner was one of two men jailed for murder over the 1973 firebombing but authorities have long suspected others were involved.
Counsel assisting Stephen Keim told AAP Finch died earlier this year and he had been due to give evidence to the inquest electronically.
His own health issues and the pandemic prevented him returning to do so in person.
The only other person convicted of the club murders was John Andrew Stuart, who died in 1979.
Finch spent his final years in his homeland after serving 15 years in Brisbane's Boggo Road Gaol before being deported.
Rumours have persisted about the involvement of others in the bombing, which saw two 23-litre drums of fuel thrown into the foyer of the club.
It was Australia's worst mass murder case until the 1996 Port Arthur massacre.