An investigation into the murder of three friends while on holiday 46 years ago is gathering “extensive” evidence ahead of hearings into the cold case, a coroner has heard.

Karen Edwards, 23, Tim Thomson, 31, and Gordon Twaddle, 21, left Alice Springs with their dog, Tristie, for a motorcycle trip to Mount Isa on October 2, 1978.

The trio were found dead with gunshot wounds in remote bushland at Spear Creek, 12 kilometres north of Mount Isa.

Queensland coroner David O’Connell was given an update on preparations for an inquest into the deaths, which is due to start hearings in October.

Counsel assisting Amelia Hughes said parties involved would receive a copy of the brief of evidence and witness list in September.

“It’s quite an extensive brief. That process is still underway with investigations ongoing,” she said on Tuesday.

Edwards’ sister and other family members listened remotely to the coroner’s pre-inquest conference by phone.

Thomson and Twaddle were family friends from New Zealand and shared a passion for motorcycles.

Edwards was Thomson’s girlfriend.

Once they reached Cairns, the friends planned to head south to Melbourne for Christmas with family.

They were last seen alive leaving the Moondarra Caravan Park in Mount Isa in a brown and white Toyota Landcruiser station wagon four wheel drive with a man on the morning of October 5, 1978.

Queensland police in 2019 renewed a $250,000 reward for information which leads to the apprehension and conviction of the person or persons responsible for the murders.

Former NSW prison guard Bruce John Preston was charged in April 2019 with the murders and granted bail in February 2020.

Crown prosecutors dropped all charges against Preston in July 2023.

The inquest will hold hearings over nine days in Brisbane Coroners Court from October 7.

The hearings are due to end days before the 47th anniversary of the victims being found dead.