Ross Bremner was extremely mentally unwell when he killed three people in 2016

The 35-year-old killed his mother and stabbed his father in Waikato's Otorohanga eight years ago.

He then drove to a bach about 60 kilometers away - before killing elderly couple Mona Tuwhangai and Maurice O'Donnell and ending his own life.

A coroner's report's found Bremner's schizophrenia quickly and drastically deteriorated - impacted by slight medication changes.

It says there were no errors in his mental health care, and the unpredictable nature of his illness led to the attacks.