VINCENT O'Dempsey previously warned a man who is now a witness in his murder trial to "keep your mouth shut or else”.

Warren McDonald said the accused killer made the comment when Mr McDonald was due to give evidence at a 2014 Crime and Corruption Commission hearing into the disappearance of Barbara McCulkin and her daughters Leanne and Vicki.

The prosecution witness told the Brisbane Supreme Court jury yesterday that he did not lie about Mr O'Dempsey's alleged 'I killed the McCulkins' confession.

On Thursday Mr McDonald claimed that Mr O'Dempsey said: "They'll never catch me because they'll never find the bodies”.

The McCulkins were last seen at their Dorchester St, Highgate Hill, home on January 16, 1974.

Mr O'Dempsey, a 78-year-old Warwick resident, has pleaded not guilty to three charges of murder and one of deprivation of liberty.

Defence barrister Tony Glynn cross-examined Mr McDonald for about two hours yesterday afternoon.

Mr McDonald told the court he did not talk about Mr O'Dempsey at the CCC hearing because "I feared for my life”.

Mr McDonald said he was worried because the CCC witnesses were supposed to remain secret but Mr O'Dempsey knew he was scheduled to give evidence.

"He told me the CCC were rounding everybody up,” Mr McDonald said of meeting Mr O'Dempsey in the street in Warwick in 2014.

"(Mr O'Dempsey said) 'I heard that they raided your place too'.

"He told me 'Keep your mouth shut or else'.”

The court heard Toowoomba police were investigating Mr McDonald over drugs in late 2014.

He struck a deal regarding jail time in return for telling them what he knew about the McCulkins' disappearance.

"You knew unless you came up with something about the McCulkin murders, the police would not be interested in doing a deal,” Mr Glynn said to Mr McDonald.

"The detectives made this very clear.”

Mr McDonald replied: "That's not correct”.

When he went to court on the drug charges, Mr McDonald was facing a 4.5-year prison sentence with parole after 18 months.

However, he received a suspended sentence and walked free.

Mr McDonald said he did not "invent the admission” by Mr O'Dempsey.

"No sir, I'm here to tell the truth,” Mr McDonald told Mr Glynn.

Mr O'Dempsey's former wife Julie Fenton said the defendant disappeared in 2014 after a renewed police focus on the McCulkins' disappearance.

"He went from seeing the kids regularly to dropping off the face of the earth without a word,” Ms Fenton said.

Mr O'Dempsey's trial has been separated from that of his co-accused, Gary Reginald 'Shorty' Dubois.

The Crown alleges a "suspected connection” between the Torino and Whiskey Au Go Go fires in 1973 "would provide a motive for Mr Dubois and Mr O'Dempsey (as a friend of Dubois to) keep Barbara McCulkin quiet”.

"It may not sound a sufficient motive or even a sensible one, but there never is for murder,” prosecutor David Meredith told the jury during his opening address last week.

The trial before Justice Peter Applegarth continues. - ARM NEWSDESK