Judge Michael Crosbie said the circumstances of the case and the man's background were "extraordinary''.
He sentenced Leckie to nine months supervision and 100 hours community work.
The court heard how the defendant was granted accident compensation following Ross' death. That sparked an investigation by the ministry into Leckie's living situation in the previous years.
"It was established that the defendant commenced living in a de-facto relationship with his partner Karin Ann Ross from July 1, 2012 and this continued until December 1, 2015,'' court documents said.
Ross was killed on December 2, 2015.
Leckie was interviewed by the ministry in February last year when he acknowledged the relationship.
"He further admitted he should have come clean with Work and Income,'' the summary said.
But lawyer Bill Wright said it was not a case of his client deliberately misleading the authorities.
He lived 11 houses away from Ross in the same Green Island street and they had come to an arrangement where he would spend the night at her house to look after her two children while she worked as a cleaner. Leckie would sleep in her bedroom but in a different bed to her and would go back and forth from his home, Wright explained.
The pair never had an intimate relationship, he confirmed.
"It was almost like a mutuality of friendship,'' Wright said.
To Leckie, Ross was "a combination of a motherly, sisterly, partner figure'', he told the court.
Wright said he had been confident of defeating the benefit-fraud allegations at trial but his client had wanted to take the blame and get on with his life.
It was certainly not a case of the pair living a lavish lifestyle, he stressed.
Wright also emphasised the traumatic turns in his client's life.
"In 50 years of practice it is one of the more tragic cases I have been associated with,'' he said.
Leckie was on a sickness benefit for four years, jobseeker support for two and a-half years, accommodation supplement for six and a-half years and disability allowance for five years, the court heard.
He admitted signing three forms which falsely stated he was not living with a partner and was overpaid $32,508.
The court heard he had since repaid the sum, following the sale of his house.
After Ross' death, he took up the care of her two teenage children.
Judge Crosbie accepted Leckie's offending was "perhaps more haphazard than overtly criminal'' but noted he had four previous similar convictions from 22 years ago.
Merritt was jailed for life with a minimum of 12 years when he was sentenced in the High Court in February for Ross' murder.
