Krishna Naidu was stabbed 15 times before he died. Photo / Supplied

Krishna Naidu was stabbed 15 times before he died. Photo / Supplied

A teenager too insane to stand trial for murder was told by his family doctor that he wasn't depressed and to exercise and play more sport - two weeks before "voices in his head" ordered him to stab a man to death.

On January 9, Tiare Towihi Nathan was taken to the doctor by his grandmother, who was alarmed by his behaviour.

The then-16-year-old was eating food off the floor, talking to himself, waving his arms in the air and sleeping in the driveway.

He had been living with his grandmother for a few months, after moving from his father's house in Northland. His mother killed herself when he was 8 years old.

The GP who examined Nathan found he wasn't talkative but admitted taking drugs. The doctor's advice: to exercise more and play sport.

Just over a fortnight later, on January 25, Nathan walked into a superette in Finlayson Ave, Manurewa, and stabbed 22-year-old student Saishwar Krishna Naidu 15 times with a sharp knife he'd taken from his grandmother's kitchen.

Mr Naidu, an electrical engineering student at Manukau Institute of Technology, was working in his parents' store.

His father, Hari Raj Naidu, told the Herald from Sydney last night that if the doctor had referred Nathan for psychiatric assessment, his son would still be alive.

"That is where the problem started," Mr Naidu said. "If he was acting strangely, or suffering mental problems, the doctor should have picked it up."

The fact that the tragedy could have been avoided made him more sad than angry, Hari Raj Naidu said. "One misdiagnosis cost my son his life."

Mr Naidu and his family left New Zealand and went to Sydney after the killing because they couldn't go back into their shop.

"I had no courage to go back to the superette," said Mr Naidu. "I sold it three weeks later for peanuts."

The family came to New Zealand from Fiji to make a fresh start after the George Speight coup in 2000.

The doctor who examined Nathan did a depression test because his main concern was whether the boy was a suicide risk.

The test result did not cause him concern, so he did not refer Nathan for psychiatric assessment.

His view of Nathan was that he was "a teenager going off the rails a little bit and trying to assert himself as an adult", court documents obtained by the Herald show.