Rhys Thomassen was desperate for help when he was admitted into the mental health facility, his father says.

A young man had told staff of a beleaguered mental health facility that he was having suicidal thoughts, hours before he was granted leave.

Less than a day after he disappeared from an escorted walking group, the body of 24-year-old Rhys Thomassen was found near Hamilton's CBD.

Stuff understands police are investigating the possibility that another patient assisted with Rhys' suspected suicide.

Father and next of kin, Ray Thomassen, told Stuff he doesn't understand why his son was granted leave on November 11, despite staff being aware that he was a flight risk, and had been increasingly troubled by suicidal thoughts in the days before.

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In a statement, the DHB said an investigation into the death was ongoing and had been referred to the Coroner.

"We are continuing to work closely with the family and our thoughts remain with them following the loss of their family member."

CHRISTEL YARDLEY/STUFF Rhys' Thomassen's father is calling for an independent review into the Henry Rongomau Bennett Centre (file photo)

It could not answer further questions about the circumstances, citing the DHB's requirement to protect patient privacy.

In a statement, police said an investigation into Rhys' death was ongoing but would not divulge the nature of the investigation.

Left with his son's death note and a bagful of belongings, Ray feels like his son was failed by the facility.

Almost three months later, he's speaking about his son's death, in the hopes that other families will be saved from the same heartbreak. He's been supported by Jane Stevens and Dave Macpherson, the parents of Nicky Stevens, who died under strikingly similar circumstances in 2015.

SUPPLIED Rhys Thomassen, 24, died after escaping the Henry Rongomau Bennett Centre in November 2019.

Ray is calling for an independent review into the facility.

It follows reports that a mentally unwell man went missing, after disappearing from the same facility on January 23.

Rhys had been severely depressed and suffered from severe and ongoing headaches, which sometimes caused him to screw up his eyes in pain. He was admitted to HRBC in October, following violent outbursts, self harm and a suicide attempt.

"To be honest, he wanted to go to Henry Bennett for a long time," Ray said. "He wanted help, he desperately wanted help.

"But he just went backwards. He just went downhill, as soon as he walked in that front door."

In the lead up to Rhys' departure and subsequent death, his notes - sighted by Stuff - documented increasing suicidal ideation. He had told staff about a suicide note he'd written and other patients told staff Rhys' had plans to kill himself. Rhys denied this when questioned.

But those warning signs weren't escalated and Ray says he wasn't made aware how downhill Rhys' mental state was spiralling.

CHRISTEL YARDLEY/STUFF The Henry Rongomau Bennett Centre is a mental health facility run by the Waikato DHB (file photo).

The raft of red flags made those notes hard to read, Ray said.

About two weeks before his death, Rhys had ran away from group of patients while on escorted leave. He was found at the bus station after a call from his father and subsequently returned to HRBC.

"He told them, if you let me out again, I'm going to do it again ... I don't think they believed what he used to say."

The day of his final disappearance, Rhys ran away from the walking group of six or seven patients, chaperoned by two psychiatric staff.

The staff ran after him, but once off the DHB premises, staff legally aren't allowed to detain patients.

In a statement, a DHB spokesperson said leave was an important element of care to support rehabilitation and reintegration.

If a patient chooses to depart, a "clear process is followed", in which police and next of kin are notified.

After Rhys had disappeared, Ray wishes HRBC had made it clear that his son was suicidal and allegedly being assisted by another patient, something staff were apparently aware of. If he'd been told, he would have dropped everything to look for Rhys, he said.

Rhys been an avid gamer, who loved Dragon Ball Z so much that he got the "M" tattooed on his forehead, like one of the main characters. He loved cycling and lifting weights at the gym.

He'd been a social guy with a circle of friends, who stuck to anything he put his mind towards, Ray said.

It's not the first time HRBC has made headlines.

Recently, clinical psychologist and aunt of Matthew Prichard-Case told Stuff her vulnerable nephew had left the facility after "repeated disappearances".

In August, DHB confirmed a man had died after disappearing while on unescorted leave.

And in 2015, the body of Nicky Stevens was found in the Waikato River, after he went missing from the centre while on unescorted leave.

Coroner Wallace Bain said Stevens' death could have been avoided, had health staff listened to warning from parents.

"The circumstances of Stevens' death make it clear that our mental health system is in urgent need of being overhauled," Coroner Bain said in his report.

Nicky's mother, Jane Stevens, told Stuff it was "devastating" to hear of Rhys' ordeal, which so closely resembled her son's.

Stevens, Case and Ray are calling for an ministerial-led independent review into the HRBC, which they believe would reveal deficiencies in risk assessments, the facility's culture, staff numbers and communication with families.

Stevens believes there should also be more support for the "isolated and vulnerable" whānau, left to navigate unfamiliar processes during the hardest time of their lives.

"We only know what we know because we've come together. How many more are there out there?"