Singh wrote the letters to renowned Hawke's Bay potter Helen Mason, who died last year aged 99.
Mrs Mason was a customer of the Sunset Dairy and befriended the couple. She visited Singh in prison after the killing.
Among papers now held by the National Library, Mrs Mason wrote how she had enjoyed watching the couple's family grow through the birth of their two children after their arrival in Waipukurau in 2000.
"After the second child was born and Suguna went back to work, I often found Chander stacking the shop with one hand and cradling the baby with the other," she wrote.
"He was a devoted father and became expert at feeding him when Suguna was forced to wean him."
Mrs Mason wrote of her surprise and concern when Mrs Chilamakuru told her the children were going to live with Singh's parents in India.
"When I asked why, she said she wanted them to learn the culture and language properly ... the baby could at least have been left with his father who looked after him so well," she wrote.
"Each time I visited the dairy after the children had gone, I could see how depressed Chander was, but I was shocked when the tragedy occurred."
In letters and Christmas cards sent to Mrs Mason from 2004 through to 2008, Singh wrote of his love for his children and wife, the success of his treatment and his plans once he was released from care.
In a June, 2008 letter written from Porirua Hospital's Tane Mahuta forensic unit, he wrote: "With God grace I will be staying for one more year and then get released.
"On release go to India for 1-2 years and come back to New Zealand with my childrens. I want to give good life to my childrens."
Three months later he wrote about becoming a blood donor and signing up for a first-aid course.
"I am happy to give blood to save people. One time's blood collected is sufficient to save 3 people's lives."
He wrote his children were "doing good and they will ask me all the time when I ring - when you are coming to India, My answer - Soon."
It was revealed last month that Singh had fled the country last October using the Indian passport issued in 2009 despite special patients being banned from international travel.
The Ministry of Health said it believed he was now in India and his "premature departure" was prompted by a "strong desire" to be with his family following the recent death of a close family member.
The ministry said Singh was the only special patient it was aware of who had fled the country since current legislation dealing with people found not guilty of crimes on the grounds of insanity came into force in 1992.
Any attempt to extradite him back to New Zealand was likely to be unsuccessful.