Local commander Superintendent Wayne Humphrey says a three-year-old child also found at the scene was covered in blood but physically unharmed.
Officers were called to a home unit in Crebert Street, Mayfield, about 10.40pm on Friday in response to reports a man had broken into the flat.
They arrived to find a 21-year-old woman critically injured lying on a landing outside her home.
Despite the efforts of ambulance paramedics, she died at the scene.
Supt Humphrey told reporters on Saturday the woman had suffered "significant, significant injuries".
"I've spent 20 minutes or so watching the body-worn (camera) footage (obtained by attending officers)," he said.
"It was an horrendous scene, horrendous."
Supt Humphrey said the young child appeared to have been present at the time of the incident and was being cared for by relatives.
A 22-year-old man arrested at the unit is in custody.
According to media reports, he was the woman's former partner and was recently released from jail on parole.
A crime scene has been established with forensic specialists on site.
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