Two teenage girls in Western Australia have been sentenced to life in prison for killing a friend to see whether they would feel remorse at the deed.

The girls, aged 16 at the time of the murder, strangled Eliza Jane Davis then buried her under a house.

They told police they knew it was wrong to kill but it "felt right", and they did not regret Davis's death.

Perth Children's Court president Denis Reynolds said the murder was "gruesome and merciless in the extreme".

Shallow grave

The pair, who cannot be named because of their age, killed 15-year-old Davis while the three were staying at the same house in the coal-mining town of Collie, south of Perth, on 18 June 2006.

They had been discussing how neither would feel bad about committing murder when they decided to kill Davis, who was sleeping in another room, a court heard in April.

They dressed in old clothes then strangled Davis with speaker wire and buried her body under the house, the court heard.

The girls confessed to police after deciding the grave was too shallow and that they would inevitably be caught.

Earlier, they had reported Davis as missing and pretended to help with the search for her body.

Their lawyers said experts were baffled as to the motivation behind the attack.