Morgan Grant appeared in the High Court at Christchurch on Wednesday morning via audio-visual link, where she entered the plea.
The details of the charge are suppressed.
Leman died in Rangiora in April last year.
Leman's torso was found in the boot of the car. His other body parts - including his head - remain missing, The Press reported.
Grant pleaded guilty before Justice Cameron Mander. A sentencing date is yet to be confirmed.
Leman was last seen alive in Rangiora on April 11.