In a statement, sister Donna Donnelly said she was relieved Tamaka had been held to account for his "senseless and heartless act".
She described her sister as a beautiful and intelligent person with nothing but good in her heart.
"She always maintained a childlike innocence about the world and never lost her dreams. We will remember her as the bright, outgoing and social person who loved karaoke, creating art work, travelling and 10-pin bowling."
Mary and her family were proud of her achievements, including a certificate in art and medals from the Special Olympics.
"Mary brought nothing but light and happiness to my life and to the world."
At sentencing, Crown prosecutor Emma Light read aloud from Donna's victim impact statement. She said the murder had destroyed the family, and Mary's elderly mother, aged in her 70s, could no longer talk about her.
A victim impact statement from cousin Helen Moore, also read aloud by Ms Light, said the family was reeling in disbelief that someone with special needs would be killed in such a brutal manner.
The "evil" murder would haunt them forever, and no words could explain the "sheer, cold horror" the family had been through.
Justice Young said Tamaka had offered no explanation for the murder.
"It is no exaggeration to say that this was a shocking murder. It is hard to understand why you killed this young woman."
Tamaka had lived next to Ms Donnelly in 2011 and apparently followed her home after seeing her at the Upper Hutt train station on August 29, 2012.
Her strangled body was found the next day.
Justice Young pointed to aggravating features, including Ms Donnelly's vulnerability due to her intellectual disability, and a "sexual element" to her killing.
A psychiatric report painted a bleak picture of Tamaka's troubled past.
He was deprived of oxygen at birth, which had left him with a degree of intellectual impairment. He had also suffered violence, sexual and emotional abuse, and had abused substances.
Tamaka had a history of previous offending including dishonesty, indecent assault and injuring with intent.
Defence lawyer Kevin Preston said Tamaka was motivated to change, and regretted what he had done.
Speaking outside court, Detective Senior Sergeant Dave Thornton said it had been a very emotional case, and police were satisfied Tamaka had pleaded guilty to the "horrendous crime".
