The 21-year-old woman was alone in her Waverley, Dunedin, home studying for her exams when confronted by her attacker about 2pm on Wednesday.
She had earlier phoned her mother after becoming alarmed at noises in the house, Detective Senior Sergeant Steve McGregor, of Dunedin, said.
Her mother, who hurried home from work, found her unconscious in her bedroom with a serious head injury, and the weapon nearby.
Det Snr Sgt McGregor would not say what the weapon was because it could help police identify her attacker.
The woman was in a stable condition in Dunedin Hospital, but could not remember much of the attack.
She was not likely to be discharged for another two days.
While she was unable to remember the attack, police had to move forward with the slower method of forensic testing, such as taking fingerprints and DNA samples from items inside the house, including from the weapon used against the woman.
The family was not yet able to return to the Scobie Rd house, which remained cordoned off yesterday while police and forensic scientists continued the search for clues.
