A teenage girl who deliberately lit a fire which killed Pukekohe mum Lynette Chapman has changed her plea on a murder charge to guilty.

The girl, 16 at the time of the incident and now 17, became tearful as she admitted the charge of murdering Ms Chapman, 49, who burned to death in a fire at her house on January 20 last year.

The girl, who had denied the charge throughout last year, also admitted two charges of arson when she appeared at the High Court in Auckland today.

Her plea came a week after a co-accused, a 19-year-old man, was jailed for two years after admitting a charge of manslaughter.

He had indicated he would testify against the girl at her scheduled trial.

The agreed summary of facts says that the girl and her co-offender went to the home, placed a t-shirt against the front door and tried to light it, but their efforts failed.

They then went back to the girl's place and returned with a sweatshirt. The male offender stayed at the bottom of the driveway, acting as a lookout as the girl managed to get in the door.

She then set fire to the sweatshirt, left it at the bottom of an internal stairwell and then left the house.

Firefighters found the house fully engulfed in fire and needed ladders to get to the upstairs section, where they found the body of Ms Chapman.

A pathologist later said Ms Chapman died of "the effects of fire".

The summary of facts said the girl admitted she heard the television playing upstairs when she approached the house on the second occasion, and that she knew Ms Chapman was home.

However, it said there did not appear to be any significant animosity between the girl and Ms Champan.

Police inquiry head Detective Inspector Mark Gutry said the girl's decision to change her plea came after her co-accused's sentencing last week.

"It's good to get an acknowledgement from her of what has occurred," he said.

Mr Gutry said he had spoken to Ms Chapman's family, who were pleased there wouldn't need to be a trial.

The girl, who turns 18 next week, will be sentenced in July.

Both prosecutors and the girl's lawyer Clare Bennett agreed to name suppression continuing.

Ms Bennett said the girl had been in the at-risk unit at Auckland Women's Remand Prison since yesterday.

Justice Geoffrey Venning told the girl her name was likely to be made public at sentencing.

The co-accused 19-year-old was given a major discount for his willingness to testify in the girl's scheduled trial when he was sentenced last week.

Justice Chris Allen said then that the boy was a suitable candidate for home detention and granted him leave to apply if he could find a suitable address.