Parents and children realised something was wrong when they saw 11-year-old Luke Batty collapse while practising in the nets with his 54-year-old father.
Witnesses called an ambulance to help the boy as junior practice was winding down at the Tyabb cricket ground, on the Mornington Peninsula.
But it became clear to those still at the oval that his injuries were not an accident when the father threatened paramedics with a knife, police said.
It is believed the man had struck his son in the head with a cricket bat before stabbing him on Wednesday.
When four uniformed officers arrived minutes later, the man shouted "shoot me" and was sprayed with capsicum foam to no effect.
He refused orders to drop the knife and rushed at one the officers until he was shot once in the chest, police said.
The father then fought with paramedics and police as they tried to take him to hospital, where he later died.
"We've had an absolute tragedy here," said Victoria police commander Doug Fryer.
There were no signs of an argument between the boy and his father before the attack. But police believe the father had planned the boy's killing and wanted to die in a suicide-by-cop.
He was known to have a violent temper and a history of problems.
Detectives said he lived in Chelsea Heights and would have needed to drive 30km to get to the Tyabb oval, all the while armed with a knife.
The boy's mother was at the oval when her estranged husband killed their son. Rosie Batty has recalled how her son had even asked to spend five more minutes playing with his father, and she didn't have any reason to be concerned.
"The only two people who love their son more than anyone is their mother and father," she told News Corp Australia. "No one loved Luke more than his father. No one loved Luke more than me - we both loved him."
Victoria's rollout of stun guns is continuing, but the four officers at the scene on Wednesday did not have Taser training or access to the electro-shock weapons, said Police Association secretary Greg Davies.
Officers used a Taser in Ballarat to stop an armed and enraged teenager on the same day as the fatal police shooting.
"[The officers in Tyabb] were placed in a horrific situation," Davies said. "What do people expect them to do? He's already killed his own son and there's potentially dozens of other children in the vicinity."
Gill Metzen, who knew Luke when he attended the Tyabb Childcare Centre for about three years, said: "He was just a bundle of joy, he was a nice little kid."
She said the father was known to have problems and she believed he was homeless and living in his car for a time.
Luke was in Year 6 at Flinders Christian College, which has brought in extra counsellors and chaplains.
