Ms Taufale's teenage son woke to find his mother's body the next morning, Justice Young said.
Smith's plan unravelled when he realised he had lost his car keys and stole Ms Taufale's car to drive back to Karori, where he lived.
He also left behind a hood he wore during the attack, Justice Young said.
Outside court Ms Taufale's uncle Ralph Jorgensen read a statement on behalf of the family.
"What happened to Mandy was absolutely shocking for the family and something none of us ever expected to happen."
"In regard to Ernest Smith the family is relieved that he finally entered a plea of guilty, and although the family cannot forget what happened, today's sentencing means they can now put this very dark period behind them and get their lives back together again."
The ordeal had been "harrowing", he said.
The two boys were being supported by the family.
"The family is hugely supportive of the children and always will be."
Ms Taufale's family thought the length of the sentence was appropriate, Mr Jorgensen said.
The officer in charge of the case, Detective Senior Sergeant Mike Oxnam, said during the investigation it became apparent Ms Taufale was a "loving, caring mother and a wonderful woman".
"For her to be taken away at this time of life is just absolutely tragic."
Justice Young said the attack was planned to coincide with Ms Taufale's vulnerability as she slept.
"This was a killing without mercy."
Crown prosecutor Grant Burston said it was a "particularly brutal and callous murder."
Defence lawyer Barbara Hunt said Smith was deeply remorseful and was still struggling to come to terms with the enormity of what he had done.
Ms Taufale had a bachelor of applied science in psychology, and had worked with disabled men and women, disadvantaged youths and victims of domestic violence.
She had volunteered at Women's Refuge, and in the two years before her death had taken up a job at Industrial Research where she worked as a science support coordinator.
