Confronted with circumstances she described as "exceptional and compelling", Deputy State Coroner Elaine Truscott ruled on Friday she was "satisfied it is in the interest of justice" that Mr Atkins be compelled to take the stand at the inquest into Matthew Leveson's death.

In a detailed and complex legal decision, Ms Truscott said her statutory duty to record findings as to Mr Leveson's death would be frustrated if Mr Atkins did not give evidence.

She concluded he had "not given a full account of what he knows about what happened to Matthew" and was "likely to be able to give important evidence about the manner and cause of Matthew's death".

The decision means Mr Atkins, who was the last known person to see Mr Leveson alive, will endure five days of questioning under oath over his knowledge of Mr Leveson's death when he takes the stand on October 31.

Mr Leveson, 20, was last seen leaving Sydney's ARQ nightclub with Mr Atkins, then 44, about 2.11am on September 23, 2007.

Mr Atkins was charged with murder over Mr Leveson's death in 2008, but exercised his right to silence during his 2009 Supreme Court trial.

He was acquitted of both murder and manslaughter.

Though the coroner's decision effectively brings to an end Mr Atkins' resolute silence, his evidence will be under the protection of immunity, meaning it cannot be used against him in any future criminal proceeding, in accordance with s61 of the Coroner's Act.

For the Leveson family, Friday's decision was a hollow victory in their nine-year search for answers as to what happened to their son and where his body is located.

"This is what the system has forced us to do. We had to decide whether we wanted to bring Matt home or get a murder conviction," Matthew's father Mark Leveson said outside the courtroom.

"We've always said from the start we want to bring Matt home."

In her published findings, Ms Truscott detailed the evidence connecting Mr Atkins to Mr Leveson's death, including the fact that he was captured on CCTV footage purchasing a garden mattock and gaffer tape at a Bunnings Store about midday on September 23, about 10 hours after the couple left the nightclub.

When interviewed by police, he denied having gone to Bunnings.

Two days after he was reported missing, Mr Leveson's car was found parked at Waratah Oval in Sutherland. Inside the car, police found a Bunnings Warehouse receipt, containing Mr Atkins' fingerprint, for a mattock and gaffer tape, the inquest heard. A large speaker system had been removed from Mr Leveson's car and was found in the couple's garage by police.

Ms Truscott said there was "no alternative person of interest" but the prospect of Mr Atkins being tried again for murder was "extremely remote" as there had been no fresh and compelling evidence since his acquittal and the police investigation had been exhausted.

Among the challenges weighed by Ms Truscott was the possible damage to confidence in the justice system by shielding a witness from prosecution for murder, which was argued by Mr Atkins' lawyer Claire Wasley.

Ms Wasley also submitted that any cross-examination of Mr Atkins which suggested he was responsible for Mr Leveson's death would undermine the jury's 2009 verdict of not guilty.

However, Ms Truscott ruled the inquest was not bound by the adversarial rules of a criminal trial and was "not concerned with whether Mr Atkins has committed a criminal offence".

"It is finding of truth, in this inquest about the manner and cause of Matthew's death and not the determination of the criminal guilt which is the task at hand," she wrote in her decision.

Ms Truscott also dismissed the defence's submission that Mr Atkins' evidence would be unreliable because he had lied to police during the criminal investigation and had given his friends differing versions as to when he had last seen Mr Leveson leave their apartment in Cronulla.

She ruled the testimony would be given in vastly different circumstance this time, during which he would be under "oath or affirmation before a judicial officer in a courtroom" and with the protection of immunity.

The Leveson family are preparing to fight to ensure Mr Atkins makes it to the stand in October after Ms Wasley told the court her client would appeal against the decision to the NSW Supreme Court.

"Although they are macabre questions, we want to know why was Matt killed, where was he killed, how was he killed, where are his remains now," Mark Leveson said.

"But the main objective is to bring Matt home, to say our goodbyes and to lay him to rest," Faye Leveson added.