Crucial information was missing from key witness statements taken from police at the scene of the murders of officers Gary Silk and Rodney Miller in Melbourne's south-east, Victoria's corruption watchdog has heard.

Key points: There were substantial differences between two statements regarding the number of offenders

There were substantial differences between two statements regarding the number of offenders Jack Rush QC said statement-taking by police had the potential to pervert the course of justice

Jack Rush QC said statement-taking by police had the potential to pervert the course of justice The hearing is focused on alleged police misconduct rather than the convictions

The Independent Broad-based Anti-corruption Commission (IBAC) is holding public hearings into allegations of serious police misconduct during the investigation into the 1998 police killings.

Bandali Debs and Jason Roberts were convicted of murdering Sergeant Silk and Senior Constable Miller, but Roberts claimed he was not there on the night the men were shot dead on duty in a Moorabbin street.

IBAC heard there were substantial differences in two statements made by Senior Constable Glenn Pullin, including an original version that made no mention of Senior Constable Miller describing more than one gunman, and a second that suggested there were two.

In that version, Mr Pullin said he asked Senior Constable Miller as he lay dying, "were they in a car or on foot?" to which Mr Miller replied "they were on foot".

Vital information

Former homicide squad detective Ron Iddles, who doubts the validity of Roberts' conviction, told the hearing he had concerns about the statement-taking process.

"When I look at the key statements, conversations which are crucial don't appear until two years later or thereabouts," he said.

"This is crucial information that comes from the dying policeman."

Mr Iddles told the hearing it "beggars belief" that a description of the suspect was missing from a statement by another policeman, Senior Constable Graham Thwaites.

"That is vital information. I just can't explain why that wouldn't be in a statement," he said.

Gary Silk (left) and Rodney Miller were shot and killed in Moorabbin in 1998. ( ABC News )

Counsel assisting IBAC, Jack Rush QC, told the hearing the investigation had uncovered evidence indicating a "pattern of systemic behaviour by Victoria Police in statement-taking that is of such gravity that it has the potential to pervert the course of justice".

Some of the practices include:

Instructions being given to witnesses to remove or exclude information from their statements, including Senior Constable Miller's description of the offender or offenders

Instructions being given to witnesses to remove or exclude information from their statements, including Senior Constable Miller's description of the offender or offenders Creating a new version of a statement that purports to be the original but includes additional information

Creating a new version of a statement that purports to be the original but includes additional information If a witness statement is deficient, a replacement rather than a supplementary statement is taken and the original destroyed

If a witness statement is deficient, a replacement rather than a supplementary statement is taken and the original destroyed Deliberately not recording a witness' description of an offender, instead noting it on a separate document

Mr Rush said one police officer indicated the practice of omitting descriptions in statements was taught at the academy, while others suggested it was discussed at detective training school or taught "on the job".

"A number of witnesses have agreed that a potential reason for the practice is to use the description of the offender later in the investigation if it matches a suspect and not use it at all if it does not," he said.

Mr Rush told the hearing the handling of witness statements had significant implications for the proper administration of justice, although it was unclear if the practices were still adopted by police.

"The manner in which police themselves provide statements and evidence in major crime cases is of critical importance to the administration of justice," he said.

"The conduct being investigated involves potentially very serious examples of police misconduct in the police investigation of serious crime."

The IBAC hearing will focus on the alleged police misconduct, rather than examining the soundness of the convictions of Debs and Roberts.