Two teenage girls have appeared in an Australian court this week after they strangled another girl to death with a chain and then videoed her blood-splattered bedroom.

Jessica Ellen Stasinowsky, 20, and her girlfriend Valerie Paige Parashumti, 19, bludgeoned 16-year-old Stacey Mitchell with a concrete block, and then strangled her, a court in Perth heard.

One of the girl's was believed to be a member of a vampire cult, reports Australia's Daily Telegraph.

The pair have pleaded guilty to the murder, which they say was provoked by their obsessive need to prove their commitment to each other.

The girl's prosecutor David Dempster told the West Australian Supreme Court the girls killed Stacey because she had been "so f**king annoying" after moving in with them in late December 2006.

On the evening of the murder, the girls had given Stacey, who they had only known for three days, an alcoholic drink containing a sleeping pill. Parashumti then repeatedly bashed Stacey in the head with a concrete block. Stasinowksy then tied a belt chain around Stacey's neck while her girlfriend continued to beat her with the block, the court heard.

Once they knew she was dead, they kissed each other over the body after she died. They also recorded the aftermath on a mobile phone, which showed them mopping blood and pulling a blanket back from Stacey's semi-naked body.

The girls grinned at each other in court, which caused the judge Justice Peter Blaxell to give them both warnings.

"She is then abused by both offenders and they both mock her English accent," Dempster said.

The two-minute scene ended with laughter and Parashumti saying: "When I was beating the f**k out of her with a rock."

The pair then dumped their victim's body head first in a wheelie bin behind the house and were still discussing ways of disposing of her body when police arrived looking for the teenager, who had been reported missing by her parents on 17 December.

The girls had been angry with Stacey over a breach of confidence, but admitted they would have killed her anyway because she was annoying, Dempster said.

Parashumti's lawyer, David Edwardson, QC, said the murder appeared to be senseless, sadistic and without justification, partly motivated by a desire to dispel Stasinowsky's jealousy about Stacey, who walked around the house scantily clad in a bikini.

The court heard about Parashumti's violent upbringing, and that she did not believe anyone from a cherished background like Stacey, who had talked about killing herself, had a right to live, he said.

Parashumti had no remorse about killing Stacey but asked her lawyer to apologise for the distress her actions caused the teenager's family.

The pair will be sentenced on 7 March.