The looming inquiry into David Harold Eastman's murder conviction will cost taxpayers at least $2.5 million, with the government pledging more cash if needed.
The $2.514 million payment to be unveiled in the Legislative Assembly on Thursday marks the most significant one-off justice investment in recent years.
The cash injection from the Treasurer's advance contingency fund is more than the ACT government's financial investment in the much-touted ''blitz'' on the ACT Supreme Court lists. It dwarfs the $319,000 bill for Steven Wayne Hillier's second murder trial in 2010 - also a Treasurer's advance.
And the sum earmarked for Eastman's legal team alone is more than double that set aside this year for the Legal Aid Commission's ''expensive cases fund''.
But the bill might yet rise, with Attorney-General Simon Corbell not ruling out more money if necessary.
''Essentially it's very difficult to predict what the total cost of this inquiry will be until it is complete,'' he told The Canberra Times.
''We don't know when it will be complete, and nor do we know the full range of costs that it will incur.
''The government is obviously committed to providing the funding to do the work it needs to do, and we are providing this amount of funding based on our best assessment to date, but I certainly don't rule out further funding depending on the course and the duration that the inquiry takes.''
Eastman is locked up awaiting the fresh probe into his 1995 conviction for the murder of Australian Federal Police assistant commissioner Colin Winchester. Winchester was gunned down in his neighbour's driveway in January 1989. A jury convicted Eastman of the slaying, and he was sentenced to life behind bars, but the prisoner has always maintained his innocence.
A broad-ranging inquiry ordered by Justice Shane Marshall in August has been in limbo pending resolution of the funding issue. The Legal Aid Commission, representing Eastman, has been unable to begin properly preparing their case on its base budget. The Treasurer's advance includes $497,000 for a legal aid grant for the man's representation. His legal team expected to include Victorian silk Robert Richter, QC, former public defender Terry O'Donnell and in-house Legal Aid solicitor Helen Hayunga.
They are expected to need paralegals to help them wade through thousands of pages of documents.
Another $1.868 million will cover the broader inquiry costs including the services of acting Justice Kevin Duggan, counsel assisting Liesl Chapman, SC, the office of the Director of Public Prosecutions, court staff and ACT Corrective Services.
There is also $149,000 put away for any alterations needed in the inquiry hearing room, yet to be selected.
Eastman's lawyers have flagged an intention to call expert forensic evidence from witnesses based overseas.
They want forensic expert Jim Wallace, based in Northern Ireland to review crucial gunshot evidence with an extremely costly scanning electron microscope. The expensive forensic work is key to challenging the original findings of Robert Barnes, who's testing for the 1995 trial is believed to have cost $300,000.
Mr Corbell said on Wednesday that the costs associated with expert witnesses sought by the prisoner's legal team would need to be paid for out of the Legal Aid grant.
But the Attorney-General said the cost of witnesses deemed necessary by the inquiry judge or other parties would come from the $1.868 million instead of the grant of aid.
The inquiry is due to begin some time next year.