A man charged in connection with three unsolved Brisbane murders from 1974 isn't the killer, police say.

Barbara McCulkin, 34, and her daughters Vicki Maree, 13, and Barbara Leanne, 11, were last seen at their Highgate Hill home on January 16, 1974 and have never been found.

A 66-year-old man has been charged with drug trafficking offences, following a raid at a rural property at Warwick, in Queensland's Southern Downs, on Wednesday, which netted $300,000 in cash and 50kg in cannabis.

The cannabis was found inside large drums and the cash was found inside the house in plastic shopping bags, police say.

Detective Superintendent Steve Holahan says while the man charged over the illegal stash is of interest in relation to the McCulkins, he's not believed to be connected to the triple murder.

"At this point in time, we don't believe he's linked to the murders, but he is linked to the broader criminal network," he told Fairfax Radio on Thursday.

He said police believe the McCulkin killers were still alive.

Mrs McCulkin and her daughters were murdered 10 months after the 1973 Whiskey Au Go Go nightclub bombing in Brisbane's Fortitude Valley, which killed 15 people.

Queensland police are offering $250,000 and an indemnity from prosecution for information leading to the arrest and conviction of those responsible.

In 1980, a coroner recommended two men be charged with the murders, after hearing evidence that the pair was seen at the McCulkin home on the night the family was last seen alive.

But the charges against the men were dropped in 1981 because of insufficient evidence to secure convictions.

On Tuesday, Det Supt Holahan confirmed police were hoping to speak with a man linked to the initial two properties that were searched, describing him as "one of the significant persons of interest".

The arrested man is due to appear in Toowoomba Magistrates Court on Thursday.

A police media conference will also be held in Toowoomba.