The Royal Commission has heard submissions that many children were placed into state care for crimes as minor as truancy or stealing lollies.
Many of those children were Māori. Coster was asked if racism was an issue after witness testimony was read to him.
"It's someone's experience that I am certainly not here to disagree with, and I won't. What I can't really speak to confidently is to what extent that was reflective of police practice at the time and what was going on there," he said.
"I certainly understand that different communities come from quite different starting points in terms of their relationships with police, and there's a deep history that sits behind that."
In many instances, police attitudes often reflected public attitudes, he said, pointing to the Dawn Raids of the 1970s as an example. Last year, the Government apologised for the raids, but the policy at the time had clear public support.
Coster also admitted children would have been assaulted while in police custody, and while he did not believe it to have been standard practice, it could have been practised widely.
Under cross-examination, he and deputy Tania Kura were asked to respond to testimony of experiences in the 1970s and 1980s.
In one case, a 15-year-old was handcuffed to a chair and beaten by officers with a phone book, so it would not leave a mark. In another case, a young woman was also beaten with a phone book strategically in the stomach and sides, where bruises would not form.
The Royal Commission of Inquiry into Abuse in State Care also heard from Police Deputy Commissioner Tania Kura, who joined the force in 1987. She said while she never witnessed any incidents herself, she knew it happened.
"For me, there were some instances of knowing - not that I saw - but knowing that potentially somebody had been assaulted for a confession," Kura said.
"What I realised is that actually that is not a widespread thing that everybody did. But when you did start it's something that would surprise me, other people would talk about it."
Coster was asked why children who were abused while in care felt they would not be believed if they went to police, and why prosecution rates for abuse in state care were so low between 1950 and 1999.
He conceded there was probably a bias towards fellow Crown agents of people in positions of power.
"It certainly seems to be true that the awareness of the risk of, for example, sexual offending by people in positions of authority through this period doesn't reflect the awareness that we all have today."
Responding again to witness testimony, this time from the 1980s, he was asked about a case where a sexual assault complaint was set aside because the victim was written off by officers as promiscuous.
"That today would be completely unacceptable," Coster said about the language in the report. "In terms of attitudes at the time, I can't say whether that reflected a bias that existed generally in community or whether it was something that was a feature of the police mindset.
"But clearly you would not expect to see something like that today," he said.
Coster said there was no doubt many children were let down by police responses.
But he said the force today was vastly different to even the one he joined in 1996, and he wanted the justice system to be a last resort for children.
Coster said police culture and attitudes had changed dramatically, and continue to change.
RNZ
