Three travellers - Karen Edwards, 23; Tim Thomson, 31; and Gordon Twaddle, 21 - were found slain in the long grass near Mount Isa in 1978.

All three had been shot in the head at close range by a .22 calibre rifle, and were stripped of identification, with their pockets turned inside out.

New witnesses began contacting Mount Isa police after The North West Star published an anniversary feature on the case last year, prompting them to launch a fresh investigation into the murders.

Local detectives have spent hours pouring over old newspaper articles in attempts to discover who killed Karen, Tim and Gordon.

Detective Senior Sergeant Tara Kentwell of the cold case investigation team said police were confident the renewed focus could lead to a breakthrough in the case.

In the months leading up to their murders, the two men had been working around Alice Springs: Gordon as a cook and Tim as a teacher in the remote community of Hermannsburg, 125 kilometres west-southwest of Alice Springs.

As their time in the outback was coming to an end, Mr Thomson's girlfriend Karen Edwards flew up to Alice Springs to meet the pair and embark on a motorcycle adventure to Mount Isa, and then on to the east coast.

“Mum and dad put her on the plane to Alice Springs and she was just so excited about the trip and seeing Tim again. It was going to be a really good trip,” Karen’s sister Robyn Banks told reporters on Saturday.

Ms Edwards had been training to be a medical technologist in Victoria and had not seen her boyfriend, Tim, for some months.

On Monday October 2, 1978, the trio farewelled Alice Springs and began the 1170-kilometre journey to Mount Isa, camping at various places along the way.

Karen and Tim rode on an "expensive" red 1977 BMW motorcycle with a home-made sidecar and Gordon rode a blue 1977 Suzuki with Victorian registration.

Police said they were “particularly interested” in speaking with anyone who may have seen a male motorcyclist who befriended the group at the Outback NT rest stop of Frewena and joined them on the final 500-kilometre stretch into Mount Isa.

On the Wednesday, Mr Twaddle, Ms Edwards and Mr Thomson checked into the Moondarra Caravan Park at Mount Isa.

Their new motorcycle enthusiast friend did not check in with them, but a man was seen socialising with them the night they arrived.

The next day the trio were spotted leaving the caravan park with a man in a brown and white landcruiser.

Karen Edwards, Tim Thomson and Gordon Twaddle were never seen alive again.

Police said the man who socialised with them at the caravan park may have been a local, but detectives did not know if he was the same man the group had camped with on the road trip.

When the trio left with the man, the campsite was intact and Mr Thomson’s dog, Tristie, was tied to a tree.

Later that day, a man in the same brown and white Landcruiser was spotted back at the caravan park calling out for the dog by name.

All of the trio's gear, except for the motorcycle sidecar, had been left at the Mount Isa dump.

Mount Isa man Stan Harris and his fiancee Kirsten Streicher found their decomposing bodies about two weeks later at Spear Creek, 12 kilometres from Mount Isa.

Mr Thomson's red motorcycle was believed to have been driven around town for a month after the murder.

A then 23-year-old local Mount Isa man was convicted and fined after he was caught with the motorcycle.

The 23-year-old also owned a car matching the description of the Landcruiser seen leaving the caravan park with the three travellers.

"A two-toned brown and white Toyota Landcruiser and a number of firearms associated with the man's address were examined by forensic officers in 1978," Detective Senior Sergeant Kentwell said.

She said detectives had been speaking to the man who no longer lived in Queensland.

The families of the three victims each made pleas to witnesses to come forward on Saturday.

Ms Edwards' brother Barry Edwards described his sister as a “determined young woman” who went on a holiday with her boyfriend and never came back.

"A lot of people would have known what happened and who did it," he said.

“It is a long time to carry something like that around.

“The effects of Karen’s murder never really left us and mum and dad died without a culprit being identified.”

In a written statement, David Thomson described his brother Tim as a kind, gentle and much-loved brother and a good friend.

"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 otherwise I would probably not be writing this letter," he said.

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

Mr Twaddle's brother John said Gordon was his best friend.

"This horrible event has prayed on my mind for 40 years and I feel for the families of these three friends who were cold bloodily murdered," he said, in his own written comments.

"I feel that a result and conviction of the person (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."

"Finally, I would like to thank the Australian police for continuing to investigate this case over such a long period of time."