The mystery of who killed three friends in cold blood on a road trip in north-west Queensland 40 years ago is "solvable", detectives say, as they and the victims families launch a new appeal for information on the case.

Key points: The group's bodies were found in bushland outside Mount Isa in 1978

The group's bodies were found in bushland outside Mount Isa in 1978 Police want anyone who saw the trio on their road trip to come forward

Police want anyone who saw the trio on their road trip to come forward Despite a $250,000 reward the murders have never been solved

Karen Edwards, Tim Thomson and Gordon Twaddle were found shot dead at Spear Creek, 10 kilometres north of Mount Isa, in October, 1978.

A post-mortem examination showed all three died from a gunshot wound to the head fired from a .22 calibre rifle.

Gordon Twaddle's brother, John, said the trio were "cold bloodedly murdered".

"This horrible event has preyed on my mind for 40 years," he said.

"I feel that a result and conviction of the person [or persons] responsible for these murders would bring some peace of mind and form some closure, but it will never take away the pain of knowing how their lives ended far too soon in such a horrible way."

John Twaddle said they had a passion for motorcycles and had set out on a trip from Alice Springs to Mount Isa on October 2, 1978.

"Gordon, my brother, was not only my best friend but we had a common interest in motorcycling, which created a very strong bond between us," he said.

"It was that interest in motorcycling that found him in the wrong place at the wrong time."

Tim Thomson's BMW motorcycle with wooden sidecar for dog Tristie. ( Supplied: Queensland Police )

They were riding a distinctive blue 1977 Suzuki GS750 and a red 1977 BMW 100S with a homemade side car, for their Doberman, Tristie.

Police said the sight of three friends and their dog travelling through the Northern Territory and Mount Isa would have been memorable and have urged anyone who saw them to come forward.

Officers said they are especially interested in a male motorcyclist who befriended the group at Frewena in Northern Territory, and travelled with them to Mount Isa.

The group planned to continue down to Melbourne for Christmas that year, but that never happened.

What police know happened to the trio October 2 — They leave Alice Springs, stop at Aileron before camping the night at Ti Tree

October 2 — They leave Alice Springs, stop at Aileron before camping the night at Ti Tree October 3 — They make stops including Devils Marbles before meeting a male motorcyclist at Frewena and all camp at Barry Caves

October 3 — They make stops including Devils Marbles before meeting a male motorcyclist at Frewena and all camp at Barry Caves October 4 — They travel with the man to Mount Isa but he doesn't check in at caravan park. That night the trio are joined by the man in the Landcruiser

October 4 — They travel with the man to Mount Isa but he doesn't check in at caravan park. That night the trio are joined by the man in the Landcruiser October 5 — The friends are last seen alive leaving with the man in the Toyota Landcruiser. The man later returns alone looking for the dog

October 5 — The friends are last seen alive leaving with the man in the Toyota Landcruiser. The man later returns alone looking for the dog October 6 — The campsite is packed up and the dog is found at the Mount Isa dump

October 6 — The campsite is packed up and the dog is found at the Mount Isa dump October 24/25 — The bodies are discovered in bushland at Spear Creek

October 24/25 — The bodies are discovered in bushland at Spear Creek November 13 — A local man, 23, is arrested after being found with of Tim Thomson's red BMW motorcycle

Tim Thomson's brother, David, said in a statement he nearly went along too.

"Tim invited me to go on the trip through the centre of Australia with him and Gordon and Karen but I was unable to go. [If I did] I would probably not be writing this letter," David Thomson said.

"My parents were shattered by the murders and I still feel a numbness that I don't think will ever disappear."

The two-tone Land Cruiser

Much of the original investigation focused on the male driver of a brown and white Toyota LandCruiser, which the three friends were last seen getting into at Moondarra Caravan Park, in Mount Isa, on October 5.

The group left their bikes and the dog behind at the campsite, but later that day police said the man in the LandCruiser returned looking for the dog.

Police say the group were last seen alive leaving this Mount Isa caravan park. ( Supplied: Queensland Police )

The next day the campsite had been packed up, and the dog was found at the Mount Isa dump.

It was another two weeks before the bodies of Mr Twaddle, Mr Thomson and Ms Edwards were discovered, by a couple walking their dogs.

Police have never found the distinctive LandCruiser or the male driver.

Originally police believed the murders were linked to another unsolved murder in Western Australia, where the body of John Tzelaidis was found dead a few months earlier, also with a .22 calibre wound to the head.

But on Saturday police revealed they no longer believe the deaths are linked.

On top of the new appeal from the families, detectives have released photos of the trio and their bikes, in the hope anyone who saw them alive in early October, 1978 will come forward.

The blue 1977 Suzuki GS750. ( Supplied: Queensland Police )

Police also want people who may have given evidence previously to reconnect with them.

A $250,000 reward remains in place for information which leads to the apprehension and conviction of a person responsible for the murders.