He waited hidden in a roof for eight hours to murder his ex-partner as she slept in her Wellington home.

After she had gone to bed, Ernest Smith lowered himself into the house, sneaked into her room and, after a brief struggle, cut her throat with a craft knife.

During the struggle, the former couple's nine-month-old son lay asleep in a cot next to the bed. His mother's body was found the next morning by her 15-year-old son, who had been sleeping at the other end of the house.

SLAIN: Amanda Dawn Taufale.

Smith made several bumbling attempts to cover his tracks, but his efforts quickly came unstuck after he lost his car keys and left a string of evidence behind him.

In the High Court at Wellington yesterday, Smith, 39, a Jamaican living in Karori, pleaded guilty to murdering Amanda Dawn Taufale, 33, on November 14 last year in her home in Tawa.

He will appear for sentencing next month.

The police summary of facts shows the relationship between Smith and Taufale was violent and deteriorated sharply in the months leading up to the murder.

Smith moved out of the Tawa home four months beforehand and was allowed to see their son only while another family member was present.

At 2pm on the day of the murder, he parked his car at the Takapu Rd railway station in Tawa and walked to Taufale's home.

He took with him a blue-handled 2.7-centimetre craft knife, gloves, screws, an electric drill and a makeshift hood fashioned out of a green top fixed with a headlight.

Using keys he had copied earlier, he broke into the empty home and climbed into the roof through a hatch in the hallway.

Once there, he set up a makeshift strap so he could later lower himself silently into the house and drilled screws into the hatch, allowing him to open it from inside the roof.

He then waited for about eight hours as the family came home and went to bed.

About an hour after Taufale went to sleep, he lowered himself into the hall, walked into her room and, after a "brief but violent struggle", cut her throat.

After the murder, Smith tried to cover his tracks by fabricating evidence of a break-in.

He spread Taufale's blood around the spare room, left a window ajar and covered the sill with rice bran oil to give the false impression of trying to clear up a fictional assailant's fingerprints.

But the plan came unstuck when Smith drove to a nearby stream to dump the murder weapon and lost his car keys.

After a fruitless search for the keys, he returned to the scene of the murder, stole Taufale's car and drove back to his for a second search.

He eventually gave up looking and drove out to Makara along Takarau Gorge Rd, ending up in a remote farm, where he fell asleep.

The next day, he abandoned Taufale's car and walked for more than an hour to an Ohariu Valley Rd house, where he told residents he wanted to speak to police.

He was arrested that night.

Police found his sweatshirt in the roof of the house, his makeshift hood on Taufale's bed, and one of his bloodied gloves.

DNA tests later confirmed the blood of both Smith and Taufale on the glove, with a small tear in the glove matching a cut on Smith's left thumb.

A stack of documents linking Smith and Taufale was found buried in pine needles in Makara, about 100 metres from Taufale's abandoned car, where a second bloody glove was found.

The missing keys were found down the side of the driver's seat of Smith's car, abandoned in Tawa.

Taufale's father, Graham Redman, said yesterday that it was a huge relief that Smith had admitted murder.

"Today's guilty plea will enable us to at last move forward with putting our lives back together, as much as we can," he said.

It is understood the two children are now being looked after by family.