Former Canberra public servant David Harold Eastman will be retried for the murder of Australian Federal Police (AFP) assistant commissioner Colin Winchester.

Mr Eastman served almost 20 years of a life sentence in jail for gunning down assistant commissioner Winchester outside his Canberra home in 1989.

But in August, the murder conviction was quashed when an inquiry found Mr Eastman did not receive a fair trial.

He was released on bail while prosecutors decided whether to retry the case.

In October, Chief Justice Helen Murrell gave the Director of Public Prosecutions (DPP) a deadline of December to declare whether it would pursue the matter.

The DPP has confirmed it would retry Mr Eastman for the murder in the ACT Supreme Court.

Under ACT law, the trial must be by jury.

It is not clear whether any new evidence has emerged.

A key sticking point to the decision for a retrial was removed last month after AFP agreed to share confidential information about the case with prosecutors.

The material is thought to have related to possible mafia involvement in the crime.

Victim's family welcomes retrial

The Winchester family had welcomed a retrial, Victims of Crime Commissioner John Hinchey said, and had been looking forward to a resolution to their drawn out battle for justice.

"They've been through many years of difficulty in this case putting aside the fact that Colin Winchester was murdered in cold blood by someone," he said.

"They've had a pretty tough time, but they're a strong family and they are resolute in their desire to see some sort of finality and justice done for Colin Winchester."

But Mr Hinchey said the Winchester family was prepared for a protracted legal battle.

"It's an unpredictable case, nothing goes according to plan," he said.

"They're well aware of that, they've had many years experience and they're prepared for what lies ahead."

Time lapse 'a problem for the DPP'

Former ACT senator Gary Humphries said the issue of who killed assistant commissioner Winchester remained unresolved and needed to be answered.

"The jury will need to be satisfied that the evidence presented, in some cases 25-year-old evidence, is still satisfactory as a basis to convict somebody of a very serious crime," he said.

"A jury may not be convinced of that and that is a problem for the DPP."

Mr Humphries said the process had been immensely expensive.

"It's been one of the most expensive and longest running legal battles in Australian history," he said.

"The fact that it isn't yet resolved means that more money has to be poured into it to make sure it's done properly.

"It's unfortunate that so much needs to be yet spent to make this work, but it's in the public interest that there be an outcome of this process."

Mr Eastman has already lodged an application in an attempt to prevent any new trial.

A directions hearing for Mr Eastman's retrial has been scheduled for Tuesday.