The family of a Hamilton man found drowned in the Waikato River had laid a complaint to the police watchdog about the police handling of the case.

Nicky Stevens was under the care of the Waikato District Health Board's mental health facility, when he went missing while on an unescorted smoke break in March.

The body of the 21-year-old was found in the Waikato River three days later.

His family's laid a complaint with the Independent Police Conduct Authority.

They're concerned police didn't start searching for him until two days after he was reported missing.

They also say police didn't take seriously Mr Stevens' potential to take his own life.