An intellectually disabled man who pushed over a fellow residential home resident causing fatal injuries has been deemed unfit to stand trial.

The coroner's report said both the victim Colin Powell, 45, and his attacker had mental ages of between seven and 10, the Dominion Post reported.

They had been on an outing to Levin with two caregivers in February 2008 when Mr Powell refused to carry some potatoes from a grocery store to their van.

He put them down and asked a caregiver, "who do you think I am?" then grabbed gravel from the carpark and put it in his mouth.

The caregiver ignored him but the other resident rushed forward, pushing him in the chest, causing him to fall and hit his head.

Mr Powell died four days later at Palmerston North Hospital of "respiratory failure with a background of pneumonia and traumatic spinal injury".

The resident had been facing a charge of manslaughter, but Judge Gregory Ross said the man's disability meant he was not fit to enter a plea or stand trial.

Judge Ross' ruling was supported by coroner Tim Scott who said it was unlikely the man understood the consequences of his actions.