Chilling and incomprehensible has been the reaction to a father taking a cricket bat and a knife and murdering his 11-year-old son at the end of cricket practice at Tyabb oval in suburban Melbourne on an ordinary Wednesday night. How could this happen?

Tragically, a computer glitch played its part in a series of catastrophic events that saw Greg Anderson turn on his son.

Victoria's police chief commissioner Ken Lay revealed the role of the antiquated police computer system in the failure to arrest Anderson for outstanding warrants when police visited him on January 27.

The officers went to see Anderson over alleged assaults. But because of the ageing IT system, they were not aware of five arrest warrants out for Anderson.

One day would have made the difference. There is a two-week time lag in the computer system showing warrants. ''If police had gone to that address a day later, those warrants would have been visible," Mr Lay said.

He is not blaming the IT system failure for the death of Luke. Rather, it is one factor in a complex set of circumstances that led to the brutal killing of the boy, with his mother Rosie nearby.

"There are a number of questions … that have to be asked," Mr Lay said. "Does the current system provide sufficient visibility to the police, the courts and other parts of government to make informed decisions that will protect our community?

"Should we have been better equipped to predict and pre-empt Luke's violent death? We only need to look back at the last few years where we've seem similar occasions where fathers have killed children.

"We need to understand how we can prevent these incidents from occurring."

He is hoping that the horror could be a watershed for action on family violence.

"We know there are thousands of young Lukes out there at risk. We know there are thousands of Rosies out there at risk. We need to get better at this. We need to invest in this."

Victoria's Commissioner for Children Bernie Geary has also ordered an inquiry to determine if the system failed.

Anderson, shot dead by police after the brutal assault on his son, had a long history of mental illness and was known to the Department of Human Services.

''Quite clearly, in this case, the family had a connection to the department,'' Mr Geary said. ''And the death of Luke, with a connection to the department and its services, requires a pretty fulsome review to determine whether there were the proper services and assistance available, to see if all the links in the chain worked correctly.''

But Mr Geary cautioned against assumptions that mental illness was a common root cause of ''insidious family violence in the community''.

Rosie Batty, who has spoken with incredible courage and compassion, said her estranged partner was a man who loved his son, but had battled an undiagnosed mental illness for two decades.

The deterioration saw Anderson going from someone who could brush off losing a job to being unemployable and at times, homeless. She permitted contact with Luke because she believed he would not hurt him.

But there was also evidence that Anderson terrorised his family. Commissioner Ken Lay said Anderson's treatment of his family had been appalling, with Rosie living in fear for the last decade.

The Prime Minister Tony Abbott struggled to express his feelings. It was an "unspeakable tragedy, an absolute unspeakable tragedy and just horrific beyond words", he said.

smh.com.au, with AAP