Mr Paterson said there was conflicting evidence on why Mrs A was not put in the HDU.
She had fallen through the cracks in the bed-booking system, although expert advice was that she might not have survived, even in the HDU.
She was nursed in a side room, which appeared inconsistent with the need to monitor her closely; no individualised care plan was written, despite her range of medical conditions; she received less nursing support than required; and the medical equipment used to monitor her in the ward was either not working correctly or was unavailable.
Her ward bed was also too small, she was at risk of falling out, and her family were upset, after she had died, to find dead insects on her sheets.
Mr Paterson said: "The numerous failings in the care provided to Mrs A were caused by poor planning of a scheduled operation for a patient with significant risk factors ... nursing staff were ill-supported to care for her in a non-HDU setting."
A blood-pressure monitoring machine used on Mrs A emitted a continual beep as it had run out of paper, depriving nurses of an important warning of any deterioration, and a nurse said to ignore the beeping.
"It seems inconceivable that such a situation could occur in a modern public hospital in 2005."
It had been expected that Mrs A would also be monitored with an oxygen-measuring continuous pulse oximeter containing an alarm feature, but one was not available.
"Again, this is inadequate," Mr Paterson said.
He decided not to refer the case for possible proceedings before the Human Rights Review Tribunal, but said this decision was "borderline".
The health board said it had apologised to the patient's widower and had implemented several changes which would be audited by the Ministry of Health this month.
But Mr Paterson said in his ruling that he was not reassured by the board's response that "screens for the windows are being sourced and fitted to reduce the number of insects".