"The victim, being a significantly intellectually impaired woman, lived with her family," Judge Denys Barry of the Wellington District Court said in his notes.
Tumaialu was about 32 years old at the time but was "unknown to the family".
"In May 2009 the victim gave birth to a baby boy. The fact of the pregnancy came as a great shock to the family."
For reasons that were not explained in the judgment, police did not interview Tumaialu until 2016.
"He provided a DNA sample which matched that of the child, providing virtually incontrovertible scientific support that he was the child's father."
Police charged him with sexual exploitation of a significantly impaired person in 2018.
Tumaialu told police he could not remember if he'd ever had sex with the victim, because of his heavy drinking and drug use at the time.
He said he remembered a girl that lived in the area who looked to be about 21, and would come to his house asking for money or cigarettes.
"He did not know, in his words, 'that she was sick'. He accepted, when shown a picture by the police, that the victim was the same girl."
He denied having a sexual relationship with the victim but said it was possible a one-night stand might have happened - but he could not remember.
In a victim impact statement, the woman's mother described how the family "descended into complete chaos" after discovering the pregnancy.
She lives with the victim and the child, who is now 9 years old.
"She speaks of how a pregnancy test confirmed her fears and how she was in total shock, how the family descended into complete chaos and in fact the relationship with her partner disintegrated in the aftermath of this," Judge Barry said.
By the time it was discovered, the victim was too far along for an abortion, causing her mother to spiral into depression, alcohol abuse, and "daily exhaustion".
The boy is now starting to ask questions about who his father is.
She said the victim is not affected by the matter anymore, but her family remains "hurt and hurting".
Tumaialu has a criminal history ranging from 1996 to 2016, but not for any similar offending.
His defence lawyer said in 2008 Tumaialu's life was "mired in drugs, alcohol abuse and hopelessness, living hand to mouth virtually on the street", Judge Barry said.
He is now in a stable relationship of eight years and has been sober for that time as well.
A cultural report showed Tumaialu and his family became culturally and socially "dislocated" when they moved to New Zealand, and Tumaialu fell into "a bad crowd".
"The cultural reporter depicts him as a different person to the young man who committed this offence back in 2008," Judge Barry said.
In deciding a sentence, he said the biggest aggravating factor was not just the effect the sexual activity had on the victim, "who is probably blissfully less affected than anyone else", but how "devastating the ripple effects of that sexual activity and that pregnancy have been on the victim's family".
Judge Barry allowed discounts to the sentence for Tumaialu's change in lifestyle, the cultural factors which left him "adrift in a sea of alcohol and drug abuse and bad company", Tumaialu's acceptance of responsibility, and his guilty plea.
He sentenced Tumaialu to nine months of home detention with six months of post-release conditions.
The sentencing happened on September 29 in Wellington.
