The commissioner's ruling said the rest home should taken steps to ensure Mrs A was receiving enough fluids after she started refusing food and drink.
Its staff were stretched by a norovirus outbreak, and the home did not have the right policies in place to identify and act upon early warning signs of Mrs A's deterioration, the ruling said.
Labour Aged Care spokesman Kris Faafoi said the report reflected the "horrifying" rest home experiences faced by many older New Zealanders.
He said Northbridge Lifecare Trust did not have the resources to deal with a norovirus outbreak and provide care for Mrs A.
"Care of our older citizens is hugely undervalued. We need to need to ensure every facility has a safe number of quality trained staff. In turn they can then provide a level of care we would all expect and that older New Zealanders deserve.
"We will continue to call for adequate staffing while horror stories continue to make the headlines."
Mr Faafoi said there were 15 substantiated complaints last month about neglect, poor food standards and substandard medical treatment at rest homes in the Waikato region alone.
He said the number of people needing rest home care is set to skyrocket, with the number of over 65s tipped to increase 84 per cent by 2026.