The Health and Disability Commissioner has criticised a carer for hitting an elderly dementia patient in late 2013.

An 82-year-old woman with dementia was bashed by a carer at a private Auckland hospital because she refused to go to bed, an inquiry has found.

The elderly patient was punched, had her arms grabbed and was slapped on the thigh by the female carer in late 2013.

The staff member, a healthcare assistant, lost her job as a result and has pleaded guilty to assault over the incident.

She has been referred to the Director of Proceedings and could face further penalties, a decision by deputy health and disability commissioner Rose Wall found.

The decision, released on Monday, ruled the healthcare assistant and the hospital, CHT Healthcare Trust, were both in breach of the code of patients rights.

The healthcare assistant's name was not released.

The report said the patient had suffered from dementia for five years and was admitted to the CHT hospital for nine nights.

It said on her sixth night in respite care she and the healthcare assistant on duty were involved in an argument over her refusal to go to bed.

A witness statement provided by a co-worker said the assistant grabbed the patient's arms and pulled her into her room.

The co-worker said she then heard "a bang in the room and [the patient] shouting, 'Leave me alone, it hurts, it's sore, don't punch me'."

When the co-worker later asked what happened the patient replied: "She punched me, she pressed me and pulled my hair."

The following morning nurses found the patient had bruising on her body, including fingermark bruises on her upper arms.

She was unable to walk because of swelling on her legs and reported the injuries were because she had been "bashed on the knee".

An internal investigation was launched by CHT Healthcare Trust and the assistant was dismissed from her employment for serious misconduct.

She had worked there on a casual basis since 2007.

Three days after the abuse, the patient was taken by her family to a public hospital and was discharged the next day into the care of her family.

In early 2014 the woman died at her granddaughter's home.

The granddaughter lodged a complaint with the health and disability commissioner into the handling of the incident and described it as "a very horrific ordeal for all of us".

In her complaint she said: "A female staff member was dismissed as a result of my grandmother's bruising to her arms and that was it. No apology or sympathy for what my grandmother had been through!"

Deputy health and disability commissioner Rose Wall was critical of the hospital's handling of the incident.

"CHT staff failed to assess (the patient's) injuries adequately, and to manage them appropriately," she wrote.

"In my view, CHT is responsible for these multiple failures by its staff and, accordingly, I find that CHT did not provide services to (the patient) with reasonable care and skill and breached the Code."

She recommended the hospital overhaul its staff training for abuse and neglect.

She also urged both the healthcare assistant and the hospital to apologise to the patient's family.