A 75-year-old man with dementia charged with murdering a woman at a Kāpiti retirement village has been found unfit to stand trial.

Edmund Alan Jenkins appeared in the High Court at Wellington today and was remanded as a special patient.

He was accompanied by a nurse who talked him through the proceedings.

Jenkins had been charged with the murder of Otaki woman Edith Roderique, 70, on March 5 at Coastal Villas retirement village.

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Justice Rebecca Ellis was satisfied under the balance of probabilities that Jenkins killed Roderique - but ruled he was unfit to stand trial due to a mental impairment.

In her judgement, Justice Ellis said Jenkins and Roderique had been in a relationship together for three years and lived separately.

On the morning of March 5, Roderique was staying at Jenkins villa when he awoke to his 7.30am alarm.

After getting out of bed, he noticed a knife on the bench that had not been put away and after going back to bed, stabbed Roderique repeatedly in the chest.

At some point during the attack, the medical alert button on the bed was pushed. When the first staff member arrived, Jenkins showed her Roderique's body.

Jenkins had blood on him and said to the staff member "I did it, I did it, I did it".

He later told police he had meant to scare Roderique with the knife as a joke but she "jumped" at him and screamed.

The judgement said Jenkins recollection of what exactly had happened next was hazy but he acknowledged stabbing Roderique.

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Two psychiatrists gave evidence that found Jenkins was likely suffering from dementia and he had problems with his speech and thinking.

The doctors believed Jenkins couldn't understand the implication of the death in any meaningful way.

Doctor Justin Barry Walsh said there was not an alternative to Jenkins being made a special patient.

"There is an ongoing risk at this stage and a need to carefully assess him and understand what has driven the offending."

Roderique was visiting a friend at Metlifecare Coastal Villas retirement village in Raumati Beach when she was killed.

The much loved 70-year-old mother and grandmother was kind-hearted and would "give someone the shirt off her back", a family member said shortly after her death.