Mike King has hit out at Waikato District Health Board for seeking a new coroner to conduct "further inquries" into the death of Nicky Stevens.
King, a mental health advocate and 2019 New Zealander of the Year, criticised the DHB after the Herald revealed on Tuesday it had made the unusual request of the Solicitor-General.
Nicholas Taiaroa Macpherson Stevens took his own life while on unescorted leave from the Henry Rongomau Bennett Centre at Waikato Hospital in March 2015.
In December Coroner Bain ruled Nicky's death could have been avoided had the advice of his parents not to allow their son outside unsupervised been followed.
Bain said because of the deficiencies in Nicky's care he was able to take his own life "in the precise manner and place he previously said he would".
However, the DHB recently raised concerns with the Solicitor-General about Coroner Wallace Bain's inquest process.
In a letter to Stevens' family, the DHB said, "One particular concern was that he dismissed the views of Dr O'Connor, without giving him an opportunity to be heard."
O'Connor is a consultant psychiatrist from Sydney who was a member of the team that reviewed the DHB's care of Nicky during the period leading up to his death.
Mike King expressed his disapproval on Facebook, writing "This is an absolute disgrace!".
"Waikato DHB interim chief executive Derek Wright and board chairwoman Sally Webb should be forced to resign effective immediately!" King wrote.
"Coroner Wallace Bain is a man of great integrity and for these two muppets to insinuate that he didn't do his job properly is unforgivable.
"After everything that has gone on this DHB and how they have wasted $100,000 of taxpayers' money with all their dodgy dealings I think we can agree the only people that need investigating is these clowns.
"My heart goes out to the Stevens whānau."
Internationally-acclaimed criminal defence lawyer Nigel Hampton QC also spoke out about the DHB's decision.
In an interview with Radio NZ, Hampton said the request by the DHB and its insurers for a new Coroner's inquest was "highly unusual and driven by money".
"I've had a lot of experience in coronial inquiries and never seen the likes of this. It's extraordinary in that sense," he said.
The Coroner's Act says that an inquiry can be reopened because of fraud, rejection of evidence, irregularity of proceedings, discovery of new facts, or for any other sufficient reason.
Hampton told RNZ the DHB appeared to be applying on the grounds of irregularity of proceedings, which he was surprised about given the experience of Coroner Wallace Bain.
He was particularly concerned that the request had come from the DHB's insurer, RNZ said.
"If it was in the interests of justice being the driver I might well have a different view, but it doesn't seem to be. It is financial considerations which are the driver," Hampton said.
The Solicitor-General's office said it must consider whether it is appropriate to order a further inquiry in the interests of justice, or to decline the request.
Nicky Stevens' parents Jane Stevens and Dave Macpherson, and brother Tony Macpherson-Stevens, said they have been "left absolutely reeling".
"This has really knocked us for a six," Nicky's mother Jane Stevens said.
"We waited three-and-a-half years for the first inquest and we just can't imagine having to go through another one."
Nicky's father Dave Macpherson said the drawn out process was prolonging their agony.
"It feels like our whānau are being punished for standing up and speaking out when it's very clear that the mental health system is failing many families," he said.
The family wrote to Prime Minister Jacinda Ardern last week after being told of the DHB's complaints.
They told the Prime Minister they no longer trusted the DHB and asked for an independent mediator to be appointed to help the parties reach resolution.
"Sadly we can now have no confidence that Waikato DHB will ever handle matters concerning Nicky's death with fairness," they wrote.
"It's been four years since Nick died and in all that time we have not been able find any closure. Four years of having to relive Nicky's death again and again. We thought that post the Coroner's inquest we would be able to find some peace. How wrong we were.
"Our whānau is asking the Government to step in by mediating an agreement between us and the two Government agencies involved [Waikato DHB and the NZ Police] that will settle outstanding matters including apologies and restitution arising from the death of Nicky."