"I am satisfied that the doctor has made all the necessary and proper investigations. She has interviewed [the student] and she has had access to the reports that I have referred to, together with a considerable volume of other material," the judge said.
"In all of the circumstances, I am satisfied that the defendant was, as Dr Duff maintains, suffering from a disease of the mind to such an extent that he did not understand the moral wrongness of his action."
Police did not dispute the psychiatrist's finding.
The student, who came to New Zealand as a refugee from South Sudan at age 12, was remanded in custody in February but was later released on bail and has been living at home. He was accompanied in court by a community mental health worker.
News reports in January said he attacked Ms Singh with a bottle in broad daylight while walking near her home.
Ms Singh ran into a driveway but the attack continued, leaving her with fractured eye sockets and a broken nose. Neighbours said they thought it was a domestic dispute so did not intervene.
Ms Singh, a solo mother of two children aged 16 and 12, led about 80 people on a protest march in March. She has received counselling, but had to take three months off work on ACC at only 80 per cent of her normal pay. She and her children were left traumatised, she said.
Ms Singh said the student now seemed to be being treated as the victim.
"It seems that he gets all this attention and all the medical care and support, whereas I have had to deal with the whole thing all by myself."