A woman has pleaded guilty to the murder of her 13-year-old granddaughter on Niblett St in March.

The murder charge was put to Lorraine Smith who appeared in the High Court at Whanganui via audio visual link from Wellington.

Smith, 59, uttered a muffled "guilty" before Justice Simon France who then announced that name suppression would cease for the defendant on Wednesday morning.

The victim was Whanganui City College student Kalis Mania Smith.

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Whanganui Police discovered her body at about 8.20pm on March 15 following a phonecall from someone who said they had harmed a family member.

Lawyer Peter Brosnahan requested that a probation report be prepared for Smith who will appear in the court again for sentencing on July 19.

Smith was remanded in custody.

Levania Cameron went to Whanganui Intermediate School with Kalis and described her as the most beautiful girl in the world.

"Every morning I would wake up to a message left by her. She had a big impact on my life and will be missed by so many," Cameron said.

"She was such a beautiful girl, she had such a fantastic smile and was always cheerful. I miss her pretty face every single day."

Kalis had started attending Whanganui City College this year after a stint at Whanganui Girls' College.

City College principal Peter Kaua said what had happened was just sad.

"I had chats to her at school and she was going okay. She was kind, friendly and she was a buddy to a lot of kids," Kaua said.

"I wouldn't want to wish something like this on my worst enemy. Emphasis has to focus on making sure kids are safe and looked after."