Gregory Anderson should have been arrested weeks before he murdered his son, but remained free because of a flaw in Victoria's outdated police database.
Anderson was wanted on five outstanding warrants when he killed 11-year-old Luke Batty after cricket training at Tyabb oval on the Mornington Peninsula on Wednesday.
But a gap in the police force's computer system was one reason police were unaware there were outstanding arrest warrants when he was questioned on January 27 about several assaults.
Victoria's police chief admitted on Friday that Anderson, 54, should have been taken into custody at that time, but a two-week delay before warrant information appears on the IT system meant officers didn't realise he was a wanted man.
"If police would've gone to that address a day later, those warrants would have been visible," Chief Commissioner Ken Lay told reporters on Friday.
The state's Law Enforcement Assistance Program (LEAP) database has been criticised for at least 15 years, though little progress has been made in fixing it.
One of the database's biggest flaws was exposed in 2011 when police discovered they couldn't easily check the parole status of criminals.
Police say at least seven people were able to commit murder after breaching parole conditions because they weren't identified as parole violators in the ageing computer database.
The flaw was fixed - three years after it was first identified.
In 2005 an ombudsman report called for scrapping the police IT system entirely.
But a planned upgrade was shelved in 2011 after mounting technical problems and concerns costs would blow out by more than $100 million.
A 2012 report into police management found $30 million had already been wasted by abandoning the project.
Police Minister Kim Wells said $20 million has been allocated in the state budget to nail down the scope and scale of the IT upgrade.
Mr Lay stressed that the force's IT failure was only one piece of a family violence case involving multiple agencies.
"This is not me blaming an IT system for the death of a young boy. It is one part of a very complex issue for us," he said.
There was also no way of knowing if Anderson would have been held in custody or immediately bailed - even if he had been arrested before the murder, he added.
The boy's mother, Rosie Batty, was also aware of the outstanding warrants when Anderson showed up at cricket practice on Wednesday night.
Ms Batty said she didn't have a problem with letting Anderson play with their son that night as she wanted Luke to know his father.
Luke was then killed as practice was winding down.
"The worst thing that could have ever happened to me has just happened," Ms Batty told Triple M radio station.
"And a little boy, who was innocent, because of a selfish act from his father who wanted to die, wanted to take him with him."
Anderson was fatally shot by police after the killing.
Victoria's Children's Commissioner Bernie Geary announced an inquiry into the two deaths.
He said the review would examine if the system had failed the boy and his estranged father.
Mr Lay welcomed the move, saying 11-year-old Luke was sadly one of thousands of children who are stuck in a cycle of family violence.
But Mr Lay admits the system need improving.
"We need to get better at this," he said.