The son of an elderly woman who died shortly after a rest home failed to provide her the required care, has hit out at an investigation into his complaints about that care.
But the investigator, the Health and Disability Commissioner [HDC], said it was confident its processes were "fair and robust".
Robert Love has called on Health Minister David Clark to investigate the processes used by the office during its two-year investigation.
In an open letter to Clark, Love says the HDC has omitted crucial evidence and claims the office appears biased toward the provider.
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The HDC launched an investigation into the care of Freda Love by St Kilda Care Home in Cambridge after Love lodged a complaint in April 2017.
Freda Love, 92, died in Waikato Hospital in February 2017 after living at the Bupa New Zealand-operated rest home for five months.
Love took the case to the Disputes Tribunal, where he was awarded $10,000 against Bupa for a failure to provide a reasonable level of care.
This included Freda being found shivering under a thin blanket in a urine-soaked bed in a freezing room with the window open.
But the main problems centred around the immobile woman's catheter, for passing urine, which Bupa said they could manage but which was often leaking.
Love has criticised the HDC's provisional findings in the case saying important information he supplied to the investigation has not been included.
These included text messages from him to Bupa St Kilda management over the leaking catheter, photographs of a wound dressing, and a statement from an HDC advocate relating to a meeting between Bupa and Love.
Love claims the statement backs up his account of the meeting that Bupa wanted Freda to use incontinence products instead of having a catheter, but that Love was supported by a doctor who said his mother's treatment prescribed by a hospital specialist should not be deviated from.
He claimed other evidence was often incomplete.
For example, in the incident where Freda was found shivering in a urine-soaked bed, Love claims no mention was made of the fact her call bell was under the bed, the thin blanket only covered half her body, there had been a heavy frost that morning, or that Freda had been left that way for up to five hours.
Love also noted he had to complain to the Ombudsman to get a copy of the first expert adviser's report on the case, only to discover that contrary to assurances from the HDC that the adviser - Rhonda Sherriff - had conflicts of interest.
Love believed it was unfair Bupa was provided the HDC's full provisional findings in early August for comment, but he was only given the "information gathered" section of the report.
"Accordingly, they are provided a distinct advantage into how to influence the provisional opinion, which will become the final opinion and HDC final report. I am denied all this."
Health and Disability Commissioner Anthony Hill said the HDC was impartial and independent.
"We hear all sides of the story, consider carefully and make a decision. People will not always be happy with our decisions."
He said those parties who had an adverse finding or comment made against them were given the draft investigation with an opportunity to respond, in accordance with the Health and Disability Commissioner Act 1994.
"The person making the complaint is given the information gathered part of the report so they can if they wish comment on the accuracy of the information."
Hill said the HDC had taken legal advice on Love's previous concerns but was confident its processes were thorough, fair and robust, and had been followed in this case.
A spokesman for Clark said the HDC investigated complaints about health care services independently of health care providers, the Ministry of Health and the Minister.
"For this reason it is not appropriate for the Minister of Health to comment on or intervene in the decisions or processes of the organisation.
"Where a person is dissatisfied with an act, decision or omission of the Commissioner, they have the right of complaint to the Ombudsman under the Ombudsmen Act 1975."
Bupa chief operating officer Maggie Owens said they had complied fully with the HDC investigation.
"We believe the handling of this complaint is consistent with the Health and Disability Commissioner's publicly documented process and the requirements as set out in the Health and Disability Act.
"This process balances the interests of all parties and ensures the fairness and integrity of both the process and the outcome of the investigation."