A decision not to charge Waikato District Health Board over the death of a patient is a "licence to kill", the man's father says.
Nicky Stevens, 21, was let out on an unsupervised smoke break while on suicide watch at the Henry Rongomau Bennett Centre in March 2015.
His body was found two days later in the Waikato River.
Yesterday the police said they had carried out a comprehensive investigation but had not found enough evidence to prosecute the DHB or any of its staff.
The decision had upset the family, Mr Steven's father Dave McPherson said.
"Someone died and no one's being held to account - it sort of gives the DHB a licence to kill, or if not to kill, to manslaughter."
He had never thought a prosecution was likely but had wanted it to happen for his son's sake.
"He was in that hospital on a compulsory care order," Mr McPherson said.
"We warned the management and the clinicians in writing and in person that he was a high suicide risk - we warned of exactly what was likely to happen, and they still did nothing."
The family was considering its legal options, he said.
"Police have 20 boxes of evidence about what happened and we're going to be getting our hands on them at some stage."
Taking a private prosecution was a possibility, Mr McPherson said.
In May, the Independent Police Conduct Authority found that police failed to conduct a search until two days after the DHB reported Mr Stevens missing.
Last month, another patient at the centre also died while on unsupervised leave, the night before she was due to be discharged.
The DHB said it would resume its own investigation into Mr Stevens' death.
An internal review had been started after his death, but put on hold during the police investigation, mental health director Derek Wright said.
The DHB had already made improvements, Mr Wright said.
"Our CEO has already come out and indicated that we plan to rebuild our unit, we've already reviewed a number of policies and procedures, we've increased staffing in that area - but anything else that comes out from the review we will obviously take seriously."
Where to get help:
Lifeline: 0800 543 354
Suicide Crisis Helpline: 0508 828 865 / 0508 TAUTOKO (24/7). This is a service for people who may be thinking about suicide, or those who are concerned about family or friends.
Depression Helpline: 0800 111 757 (24/7)
Samaritans: 0800 726 666
Youthline: 0800 376 633 (24/7) or free text 234 (8am-12am), or email email@example.com
What's Up: online chat (7pm-10pm) or 0800 WHATSUP / 0800 9428 787 children's helpline (1pm-10pm weekdays, 3pm-10pm weekends)
Kidsline (ages 5-18): 0800 543 754 (24/7)
Rural Support Trust Helpline: 0800 787 254
Healthline: 0800 611 116
If it is an emergency and you feel like you or someone else is at risk, call 111.