Police are hoping to release a decision in early July on whether charges will be laid over allegations of abuse in the adolescent unit at Lake Alice Psychiatric Hospital in the 1970s.
Two previous investigations, in 1977 and 2010, resulted in no charges being laid.
The head of the Criminal Investigation Branch, Detective Superintendent Thomas Fitzgerald has told the Inquiry into Abuse in Care, the latest investigation has been wider in scope than the previous ones.
''It is thorough and we need to take that time to make sure it's done right.''
Police formally apologised to survivors on Thursday at the Royal Commission, acknowledging failings in the earlier police investigations.
Fitzgerald said the decision to apologise was made by Police Commissioner Andrew Coster and his executive after consideration and looking at what had been done over the years.
''Because of those facts, it's important police recognise what that has done to the survivors.''
He said police admitted an institutional failing and were not looking to blame anyone or saying any individuals are necessarily at fault.
Fitzgerald was asked by lawyer Karen Feint QC to speculate on how having had more resources devoted to the investigation would have changed the investigation.
''It is somewhat speculation, but with any investigation if you have further resources to widen the scope in the early stages. There is of course a chance it will lead you down different paths and widen that investigation.''
He said with the benefit of hindsight, New Zealand Police in 2021 looked back on the investigations conducted between 2002 and 2010 with regret.
''We wish now we had managed to reach a decision sooner in that investigation.
''Like all people reporting a serious offending to police that has significantly impacted their lives, the survivors of Lake Alice would have benefited from receiving a decision sooner.
"I acknowledge delays in police decision-making in a context of this type of offending can be very difficult for victims and I apologise to them on behalf of police.''
He also acknowledged the difficulty that the decision in 2010 not to lay criminal charges would undoubtedly have presented for at least some of the survivors of Lake Alice.
Fitzgerald said police prosecution decisions are based on a number of factors including defences defendants would have available to them which would reduce the likelihood of being able to achieve a criminal conviction.
''It is not unusual for police to believe a complainant, but still not be in a position to lay charges.
''Such decisions are often very difficult for investigators, however it does not mean a case has not been properly investigated or a complaint has not been treated seriously by police.''
Decisions about whether or not to lay charges often involved weighing a range of complex and competing factors, he said.
''I acknowledge, however, for any victim, including the survivors of Lake Alice, the decision not to lay charges may make them feel that police did not believe them or that police are content to let someone get away with their offending.
''On behalf of New Zealand Police I acknowledge how difficult that can be for those who have come to police in the hope of seeing a prosecution and conviction in response to traumatic events that have occurred in their lives.''