A patient who died after an incident at Christchurch's Hillmorton Hospital was left unresponsive for 12 hours before medical attention was sought, his family say.

Alain Closel was rushed to Christchurch Hospital in 2013, the same day he was found cold and pale by a nurse at the mental health facility.

Police have been investigating the incident. Mr Closel's family say his death was preventable.

The 67-year-old's son, Mark Closel, says he didn't know his father was being cared for by staff at Hillmorton Hospital "until it was too late".

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A Canterbury District Health Board report, obtained by NZME. News Service, shows staff did not take the appropriate steps to care for the patient, who was known to fall over frequently.

Said Mark Closel: "He was left on the ground and then put in a chair over a 12-hour period and wasn't given medical attention ... He was just sort of left.

"The problem with that is [finding] who's culpable. Who didn't do what they should have done over that period of time ... when you've got different staff on different shifts?"

Alain Closel's life support was switched off on May 5, 2013, as he lay surrounded by his family, his favourite song playing in the background.

Health board chief of psychiatry Sue Nightingale said the board conducted a full investigation after any serious adverse event.

"We have conducted a detailed review into the circumstances surrounding his death and have made significant changes to how we provide care, and provided additional training to staff."