Warning: Contains graphic content.

A Wellington man has admitted bludgeoning a woman to death with a steel mallet so he could sexually violate her 12-year-old daughter.

Joseph William Borton was renting a small flat in Lemnos Avenue in the high-end suburb of Karori when he met the victims, a 52-year-old woman and the girl.

On April 4, 30-year-old Borton saw the pair entering his landlord's home to use the laundry, and followed them into the house with a mallet he'd previously stolen.

The girl recognised him immediately and said hello, but as she turned to catch a house cat she was looking for, Borton swung the mallet at the back of her head, sending her flying, the summary of facts said.

"The defendant placed one hand around her throat and his other hand over her nose and mouth. She struggled for breath," it said.

The girl clawed at his hands, screamed, and tried to bite Borton's fingers, but was unable to get away from him.

He then struck her again in the head, this time caving her skull inwards and knocking her unconscious.

The 52-year-old victim heard the girl scream and came to help her, but as she entered the room Borton struck her repeatedly in the head with the mallet, fracturing her skull and causing heavy blood loss.

As she lay dying on the floor, he undressed the girl and sexually violated her, telling her "it's okay".

The girl then watched as he went over to the dying woman, who was groaning in pain. She fell unconscious again and Borton began undressing the woman.

Borton then returned to his own flat and showered, before coming back to clean up the blood from the victims' head injuries, the summary said.

When the girl regained consciousness again, Borton came into the room and told her there had been an earthquake, injuring her and her mother. He said her mother had gone to hospital and would be back soon.

He then undressed himself and lay down by the girl, saying "let's play". The girl lost consciousness again.

The next day Borton called 111 and said he had killed the woman, referring to himself as a "peeping Tom".

He pleaded guilty in the High Court at Wellington this morning to all charges, including murder, sexual violation, indecent assault, wounding with intent to cause grievous bodily harm, and several theft charges.

An autopsy revealed the woman suffered at least eight separate head injuries. They were so significant that fragments of her skull were pressed into her brain.

The 12-year-old needed surgery to reconstruct her skull.

A scene examination uncovered the bloodied mallet in a bathroom sink with various other items, including a used condom.

Borton told police he intended to "incapacitate" the 52-year-old so he could sexually offend against the daughter.

He described the sexual offending in detail.

Police found the woman's body about 1.10pm on April 5, and the seriously injured girl in the same room.

The child has automatic name suppression due to her age and the sexual nature of the charges, and the dead woman has interim name suppression.

Borton's offending also includes stealing photographic equipment worth more than $4600, and clothing. A phone seized by police contains photos of apparently oblivious women wearing the clothing that Borton later stole.

He has been convicted and given a strike warning, and will be sentenced in October.