Their mothers are close, and they and their siblings grew up together.

So when Ms Farrell invited Kotiau and his sister Kieahn to her home for a games night no one ever could have guessed he would kill her, much less what came next.

Kotiau, 23, was jailed on Thursday for up to 27 years for the murder, the details of which he has kept to himself.

He has only ever conceded that what he did to Ms Farrell behind the door of her Ballarat bedroom was "brutal".

Pathologists say she died from a head injury, either the result of several heavy blows or, more likely, one blow with a heavy object.

Kotiau and his sister arrived at Ms Farrell's home in the suburb of Canadian on February 16 last year for a games night.

They played and drank for hours and about 6am they all headed to bed - Ms Farrell gave Kotiau a bucket and bottles of water because he was feeling unwell.

At some point, Kotiau went into her bedroom and killed her. When Kieahn woke her brother told her "I'm going to show you something that will change our lives forever ... there's a body".

Ms Farrell's naked body was lying on the bathroom floor. When Kieahn said they needed to get help, Kotiau told her no.

He lined the boot of his sister's car with a blue tarp, put Ms Farrell's body inside and they drove back to their Altona home, where he picked up a hammer and chisel.

They drove through Melbourne so Kotiau could "find a perfect place". He stopped at Nambrock and carried Ms Farrell's body into a paddock. He knocked out her teeth with the chisel and hammer and set the body on fire.

The siblings stood and watched for 20 minutes while Ms Farrell's body burned.

When Ms Farrell was noticed missing, Kotiau started getting calls.

"You guys will find out. You already know," he told them. He confessed to his girlfriend that what he'd done was "brutal".

Police discovered Ms Farrell's badly burnt body in the paddock. Her teeth, part of a mandible bone and a necklace were found in a ziploc bag under an Altona bridge.

Supreme Court Justice Paul Coghlan said Kotiau's conduct was extreme.

He involved his younger sister in what he'd done and now she too would have to live with this forever, he said.

The suffering of Ms Farrell's family and friends was greatly increased by the close relationship she had with her killer, the judge said.

Family and friends were in court on Thursday.

"She loved you. She looked after you," one man shouted after the sentence.

He threw a cup at Kotiau in the dock, before launching himself at the killer.

He was pulled away, angry and sobbing, by supporters, security and police.

Kotiau will be eligble for parole after serving 22 years.

Australian Associated Press