A key witness in the case against a Dunedin doctor has told a murder trial the accused returned from the teen victim's house holding her blood-covered cellphone.

The trial for Venod Skantha, 32, enters its second week today.

He has pleaded not guilty to the murder of 16-year-old Amber-Rose Rush and four counts of threatening to kill at the outset of his trial before the High Court at Dunedin.

The victim was allegedly stabbed to death in the bed of her Corstorphine home around midnight on February 2 after the defendant was driven to the address by a teenage boy.

The Crown's key witness against Skantha has taken the stand this morning - hidden behind a screen.

The teen boy, who has name suppression, claims he drove the doctor to Amber-Rose's house the night she was killed.

The teen's police interview was played to the court this morning.

He said Amber-Rose and Skantha had an argument via Facebook messenger the night she was killed.

Amber-Rose Rush was fatally stabbed in the the throat in her bed.

"I sent him a photo of an Instagram post Amber had put up. Venod got a bit ticked off. He said 'I'm coming to get you'."

When Skantha arrived he told him to get in the car - and that the teen would be driving.

"Drive to Amber's house. I've got a master plan," the teen said Skantha told him.

"I thought he'd had too many wines. He does like his liquor."

The boy says Skantha told him to park around the corner, where he waited and listened to music.

"He was away for five to 10 minutes. Then he comes back and says "open the door. Don't let me touch it'."

The teen said Skantha was holding Amber-Rose's phone.

"It was in a clear case with her licence in it. I saw it was covered in blood. Then I saw him put one of his kitchen knives in the [car's] side pocket.

"I used to cook for him quite a lot – I knew those knives," the teen said.

"I asked him; Vinny, What have you done?" He said it doesn't matter."

The young witness says Skantha later admitted to him he'd killed Amber-Rose.

"I didn't know what I was feeling."

He said Skantha asked him to drive to nearby Blackwell quarry - where he disposed of her phone.

"Then we lined the boot with cardboard boxes. He put bags of his clothes in the boot, and shoes.

"Later he asked me to clean the shoes - but I left blood spots on them on purpose."

The shoes were later found by police - hidden in Skantha's Balclutha girlfriend's garage.

The teen says he drove Skantha to Balclutha, and back to Dunedin the following day.

He also accompanied him to meet with Lisa Ann Mills - Amber-Rose's mother.

"I just sat there thinking, 'I'm in the room with her murderer. What the f*ck am I doing?'"

Skantha and his girlfriend dropped the youth off at his Dunedin home.

On the drive back to Dunedin, he and Skantha talked about what had happened.

"[I asked him] what he was going to do with the knife. He said – 'I'll cut some chicken or something'."

"Vinny told me he cut her ear so it was dangling off. She was just about to scream - and he says he cut her vocal cords. There was a dog in there but it was bloody useless.

"Then he just walked off – left the key in the door."

When the detective left the room in his initial police interview - the teen turned to the camera.

"Whoever does see this recording – I'm deeply sorry for what's happened, I wish I could've done something."