"We will now take time to carefully review the report to understand this in greater detail,"the force said in a statement.

The families of the slain officers said they took comfort in IBAC being unable to substantiate an allegation that a key statement used in the prosecution of Debs and Roberts was fabricated.

"This is all that matters to us,"they said.

"[It means] that the content in and of itself cannot be declared as untrue. It was the process of getting the statement to court that was found to be irregular. We have the utmost confidence,faith and respect in the leadership of[Silk-Miller investigation Operation] Lorimer and all members of the taskforce."

The practice IBAC uncovered was used in other investigations by some officers in the armed robbery and homicide squads in the late 1990s and 2000s,the watchdog said,including in investigations into armed robberies in which Debs and Roberts were suspects.

Police before the commission said one of the reasons behind the practice was that witnesses could get the description of offenders wrong,and they did not want descriptions as they were first recorded to be inconsistent with the appearance of the offender they later charged.

IBAC said there was no legitimate reason for the practice.

"In essence,they did not want to initially include the description in the witness statement that could potentially undermine the chances of a successful prosecution,"the commission's report said.

The commission established there were at least eight"replacement statements"from first responders used by Operation Lorimer.

Other practices IBAC found were widespread included police falsely representing their notes as contemporaneous and officers making statements in collaboration with each other,therefore potentially contaminating the statements.

Because Victoria Police never called out the improper method,Mr Redlich said there was a risk of it occurring today.

The inquiry was started after a whistleblower uncovered an original statement never used in the prosecution of Debs and Roberts.

One of the first officers on the scene,Glenn Pullin,had made two statements but his original was hidden. The second included more of Senior Constable Miller's dying words to suggest there were multiple offenders.

The discovery of the original statement led to the allegation that Mr Pullin fabricated his account,potentially assisting Roberts'contention that he was not at the scene of the murders.

Significantly,IBAC did not substantiate the allegation that the second statement from Mr Pullin used in the prosecution contained a false account.

IBAC has recommended Victoria Police strengthen its evidentiary and disclosure practices,calling for the Victorian government to introduce a statutory obligation of disclosure.

The Court of Appeal was yet to hand down a decision on Roberts'appeal,which was heard last month. The court can acquit Roberts,order a retrial or dismiss the appeal.