A Christchurch Alzheimer's patient seen "behaving strangely" a day before his body was found floating in a river mouth had drowned accidentally, a coroner has concluded.

Rest home resident Peter Tangira, 64, was found dead by a whitebaiter in the Styx River mouth in Brooklands on November 8 last year.

The whitebaiter, Adam Blake recognised the dead man in the water as being the person he'd seen the night before acting oddly.

Mr Tangira was last seen at his Woolston rest home at around 6pm the previous evening.

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Rest home resident Peter Tangira, 64, was found dead by a whitebaiter. Photo / Supplied via police
Rest home resident Peter Tangira, 64, was found dead by a whitebaiter. Photo / Supplied via police

Reported missing at 8.44pm, he was described as suffering mild dementia but otherwise in good health.

A police search began. They were advised that Mr Tangira had disappeared from the rest home before.

That night two men saw him near the Styx River mouth and Brooklands floodgates.

Steven Preston saw him at about 5.30pm and again at 9pm when he was "pacing and staring into the water".

Mr Blake also saw Mr Tangira "crouching and talking to the ground".

Mr Tangira told the whitebaiters that he had caught the bus from Christchurch.

"At one stage, the men saw Tangira lying down by the bridge," said Coroner David Crerar in findings released today.

The next day, Mr Blake found his body in the water.

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Coroner Crerar found that Mr Tangira's cause of death was immersion with signs consistent with drowning, and in the context of his Alzheimer's disease.