The state coroner will make a ruling on Friday on the deaths of two friends in far north Queensland, in a mystery now more than 20 years old.

Julie-Anne Leahy and her best friend Vicki Arnold were found dead in a four-wheel drive parked in remote bush on the Atherton Tablelands, near Cairns, on August 9, 1991.

Michael Barnes will deliver his findings on Friday following a third inquest into the deaths.

He has the power to commit a suspect to stand trial, as the inquest was heard under the Coroners Act 1958, not the 2003 version now in place.

A court spokesman told AAP there were a number of other options available to Mr Barnes.

He could declare an open finding, deem the case a double murder by a person unknown, or rule it a murder-suicide.

The previous inquests found that Ms Arnold killed her friend by shooting her with a sawn-off rifle, bludgeoning her with a rock and slitting her throat before shooting herself.

Sharon Daley, who was a service station attendant in Atherton at the time of the deaths, gave evidence for the first time during the recent inquest.

She said a man came into the service station looking for Mrs Leahy just after 6am (AEST) on the day the women went missing, July 26, 1991.

The man has denied any involvement in the deaths.