Edward Livingstone's psychotherapist says she was naive and wrong to write a letter for court saying he was not a violent man.
The evidence was given yesterday during the third day of the inquest, in Dunedin, into Livingstone's shooting of his children, Bradley and Ellen, 15 months ago.
Edward Livingstone saw the psychotherapist - whose name is suppressed - 24 times in 2013.
After just six weeks, she said he came to her asking for a letter to show his lawyer how far he had come.
In the letter, she wrote he was suffering from depression but was working hard to change himself and was no danger to his ex-wife and children, only driven by his love for them.
Livingstone produced this in court in response to serious breaches of a family protection order, in which he had emailed and called his ex-wife Ms Webb at work and gone to the house.
The letter appears to have been one of the pieces of information relied on by the judge who allowed a police diversion and, for a second breach, later discharged him without conviction.
Taken through this letter under cross-examination, the therapist admitted it was based on no independent information but based on her feelings and Livingstone's own reports.
She said she had been naive, had reformed her opinion about him not being violent, and wrongly took it for granted anyone reading the letter would understand it came from a therapeutic context.
She said she would not write such a letter again.
Livingstone seems to have been selective about who he told what.
He told a police officer about historical arson and assault charges, but never his therapist or any of his doctors.
They said that would have raised a red flag.
He told a neighbour and a virtual stranger about his plans to kill his family but no-one who had power to influence his situation.
A senior mental health expert, Dr David Chaplow, reviewed Livingstone's psychiatric care and found the medical team had failed to collect much of that sort of important information.
Even so, Dr Chaplow gave a warning to the coroner not to leap to simple conclusions putting all the blame on state or other agencies.
"I would remind the court that he had capacity to make choices and, while there may be many agencies that did not meet their standards, in a sense the final decision was his."
The inquest is expected to finish this morning.