Murderer Vincent O'Dempsey told a friend he was "screwed" if a man jailed for the Whiskey Au Go Go firebombing gave evidence against him, an inquest has heard.
O'Dempsey was prepared to have James Finch - who was deported to the UK after being convicted along with John Stuart for the 1973 deadly blaze - "knocked" or killed if he ever returned to Australia, the Coroners Court in Brisbane was told on Thursday.
Warren McDonald told the inquest into the Fortitude Valley bombing that killed 15 people that his former mate O'Dempsey wanted him to shoot Finch if he surfaced and to "get a notch on your gun".
Mr McDonald said via video link that his father - a former Sydney underworld figure - had received a tip-off in the early 2000s that Finch was returning from the UK to give evidence against O'Dempsey about the firebombing.
"He didn't take it well," Mr McDonald said of O'Dempsey who watched on in the court dock.
He said O'Dempsey - convicted of murdering Barbara McCulkin and her two daughters 44 years ago - made Mr McDonald travel to his father's Wooli, NSW residence to get more information.
"We went for a walk and I told him it (the tip-off) was a federal copper mate's of dad's and to take it extremely seriously," Mr McDonald told the court.
"He (O'Dempsey) said to me 'if Finch is coming back he will have to be knocked'."
Asked by counsel assisting the coroner Stephen Keim what that meant, Mr McDonald said: "It is not good for Finch. He is going to be murdered.
"(But) we were waiting to see what was going to happen."
A couple of months later O'Dempsey made the almost five-hour drive to Wooli with Mr McDonald from their Southern Downs cannabis property in Queensland to personally speak with the father.
O'Dempsey again shared his concerns about Finch when they went fishing together a day after arriving, the court was told.
"My father said to Vince 'what did you think about the information?. Vince said 'if Finch comes back I am screwed - he's the only one who can finger me for the Whiskey'," Mr McDonald said.
"Dad said 'is there anything we can do to help, let me know'. He (O'Dempsey) said 'nah, we will be right, the young fella (Mr McDonald) has got a ... shot gun'."
Mr McDonald said the topic only came up once more after the trip.
"I asked him later what was happening with Finch and he said 'it has all gone quiet'," he said.
Mr McDonald has since fallen out with his life-long mate, saying O'Dempsey threatened his young family before giving evidence at a 2014 Crime and Corruption Commission hearing into the disappearance of Barbara McCulkin and her daughters Leanne and Vicki.
Mr McDonald admitted he lied under oath at the hearing but later provided evidence against O'Dempsey at the trial for the 1974 McCulkin murders, in exchange receiving a reduced sentence on a drug conviction.
The Whiskey inquest was re-opened following evidence in the trials of O'Dempsey and Garry Dubois who were convicted in 2017 of the McCulkin murders.
Mr McDonald said O'Dempsey had confessed to the McCulkin murders at around 1997-98 at another cannabis crop.
"Vince said to me 'you need a notch on your gun ... you need a kill. When I was your age I had several notches on my gun'," he said.
"He brought up the McCulkins. He said 'I murdered the McCulkins and Shorty (Dubois) raped them."
The inquest - which is expected to hear from disgraced detective Roger Rogerson on Friday - continues before coroner Terry Ryan.