A full bench of the ACT Supreme Court overturned yesterday the decision of Justice Shane Marshall, who earlier this year ruled he had no power to order a second inquiry into findings that Eastman shot assistant police commissioner Colin Winchester in 1989.

The three judges said Justice Marshall's interpretation could bar innocent people exonerated by scientific advances from rightfully obtaining a fresh review.

''The consequences of the construction accepted by His Honour [Justice Marshall] would be that if, for example, advances in forensic medicine proved the innocence of the person convicted, no further inquiry could take place,'' they wrote. And the ruling also paves the way for Eastman to again make his case before Justice Marshall, relying on fresh questions about his guilt.

Their evidence includes a witness who could explain the presence of gunshot residue in the boot of Eastman's car, as The Canberra Times revealed last year, and questions about the ballistics analysis.

But the man's legal team will have to prove there is ''a significant risk that the conviction is unsafe because of the doubt or question'' in order to secure an inquiry.

Assistant police commissioner Winchester was shot dead in the driveway of his Deakin home. In 1995 a jury found Eastman guilty of murder, and he was subsequently jailed for life, but he has always maintained his innocence.

Outside the court, Eastman's solicitor Terry O'Donnell said yesterday's ruling was only a first step.

''I've no doubt that Mr Eastman will be very pleased that the avenues to prove his innocence are still open,'' he said. ''I've investigated this matter thoroughly over the past three years and it's probably a greater miscarriage of justice than [Lindy] Chamberlain.''

Yesterday's decision hinged on 2001 amendments to the Crimes Act, which dealt with the avenues through which a convicted person can seek an inquiry.

More than a decade ago former Chief Justice Jeffrey Miles granted Eastman an inquiry focusing on his fitness to plead, but the prisoner has since agitated for a broader review.

The law allows for an inquiry only if there is a ''doubt or question'' about their guilt, and the doubt or question could not have been addressed at an earlier proceeding.

Justice Marshall ruled that because Eastman had already raised a doubt or question about the conviction he had no power to grant a further review.

The decision meant the man's last hope rested with the Attorney-General Simon Corbell, who could wield executive power to order a fresh inquiry.

But Eastman's barrister, Robert Richter, QC, argued the judge misconstrued the intent of the legislation. And, in a rare twist in the ongoing legal saga, lawyers for the ACT government agreed.

Solicitor-General Peter Garrisson, intervening on behalf of Mr Corbell, sided with Mr Richter on the question of whether more than one inquiry was allowed.

Justices Anna Katzmann, Anthony North and Margaret Sidis formally declared a person was not barred from a second inquiry merely because one had previously occurred.

And they directed Justice Marshall to rule on whether Eastman should have a second inquiry.

Eastman's lawyers have a string of grounds they will seek to put before the judge.

Chief among them is the emergency of an old friend who claims he borrowed Eastman's car to go rabbit shooting in the 1980s and left his rifle in the boot.

Eastman's lawyers will argue it could explain the presence of gunshot residue in the car.

They will also point to the high-profile British case of a Barry George, wrongly convicted but eventually acquitted of murder after doubt was cast on the reliability of ''low-level'' gunshot residue.

And they will also argue the High Court, in ruling on one chapter of Eastman's legal odyssey, wasn't told the trial judge had been aware of material raising the question of fitness to plead.