But a detective sergeant told the ACT Coroner's Court on Wednesday she stood by the decision, saying the circumstances called for the priority to be the nine-year-old's welfare, rather than a criminal investigation.

She said she believed child protection workers had a relationship with the boy, and also believed he would not trust police because of their involvement in a traumatic separation from his mother back to his father.

She said if Bradyn made disclosures of abuse to child protection, they could take steps to remove him from his father. A criminal investigation could follow, she said.

In emotional evidence given this week, the detective said she "hand on heart" would have made the same decision and that it was to try to protect the child.

She believed if police intervened such an outcome would have happened, but sooner.

"And I stand by that," she said.

Among her considerations was the reaction of the father while the boy was still in his care if police began investigating him for criminal conduct.

In one email, police refused the referral saying there had been no specific disclosures of abuse.

Bradyn did not disclose the violence to anyone, including teachers, until his mother took him to Victoria in November 2014. But a Victorian court ordered the return of the boy to his father.

Only when he heard he was to be put back in the care of his father did he reveal abuse to Victorian authorities.

Graham Dillon, 41, beat the boy to death in February 2016 after years of abuse. He is serving more than three decades jail for the murder.

An inquest is examining what various agencies, including ACT Policing and child protection, knew when, and the adequacy of their responses.

The inquest heard that in considering the referrals the officer had information that both the boy's father and mother were using ice. The allegation about Graham Dillon was later confirmed with testing.

Police also had information about a significant history of family violence by the father, including six months in a Tasmanian jail.

In her statement, the detective noted there had been no reports in the system of Dillon having mental health issues.

She noted too that reports from the school of bruising to the boy's face were at that stage only suspicions.

The detective said she did not recall seeing information that Bradyn's mother had expressed fear for the boy's safety with his father.

She also did not remember seeing information that Dillon had threatened to kill anyone who took the boy away. But she said this would not have changed her decision to refer the matter back to child protection for a welfare investigation.

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The detective said that just because the request for a criminal inquiry had been rejected, did not mean the child was left "swinging in the wind".

It was a referral back to child protection.

She said it was always open to return to a criminal investigation.

But she said a criminal investigation was secondary to the welfare of the child.

The inquest heard there were a number of instances of domestic violence perpetrated by Graham Dillon recorded on the police database.

One instance was an allegation he had thrown a knife at his partner while she was holding their infant.

That information was not reflected in correspondence with child protection, but the officer believed child protection had it available.

The inquest continues.