A High Court judge has warned a sexual abuse conviction doesn't mean a witness murdered a teen girl.

The key witness in the Amber-Rose Rush murder trial lied to police about sexual offences he committed just two months before her death, the court has heard.

Venod Skantha, 32, is accused of stabbing the 16-year-old to death on February 2 last year then later threatening to kill the teenager who drove him to the scene.

That teen has been in the witness box in the High Court at Dunedin for the last three days and defence counsel Jonathan Eaton, QC, finished his cross-examination this afternoon by raising lies he had previously told police.

The witness admitted he had been at the Dunedin Central police station in December, two months before he was being interviewed about Amber-Rose's death.

Venod Skantha is accused of stabbing the 16-year-old to death. Photo / File

He confirmed it had been regarding sexual allegations. The teenager conceded he had initially lied to the police about his involvement and some time later admitted to the crimes.

"You've lied to the police about what happened to Amber," Eaton said.

"No," the witness replied.

"You do know what a compulsive liar is ... That's you isn't it?"

"No."

"You've lied to the police and you've lied to this jury and you've done so to falsely implicate Dr Skantha?"

"No."

But Justice Gerald Nation had a warning for members of the jury about the defence's argument.

"Lying to the police is the issue here – this past offending doesn't mean he's any more likely to have killed Amber-Rose Rush."

The teen witness told the jury, when they were in Balclutha the defendant had shown him how he had killed Amber-Rose.

"He grabbed a knife and demonstrated what he did to her on me," the witness said.

"This is one thing I'll never forget, that'll linger with me for the rest of my life."

Earlier this week, the witness told the jury he had cleaned the bloody knife Skantha allegedly used to kill Amber-Rose.

But in an interview with police he said it was the defendant who had washed it.

"It must have been a harrowing experience ... watching the blood run off it down into the sink," Eaton said.

"Do you remember doing that?"

The teenager said he did but when the inconsistencies between his statements were pointed out, he was equivocal.

"I can't fully remember," he said.

It was also revealed today that a meeting organised by Amber-Rose's mother Lisa-Ann Mills on February 4, attended by Skantha, his ex-partner Brigid Clinton and the teenager was being monitored by police.

Eaton told the witness a woman who had been in the hotel room was a police officer.

The witness had told police in his interview that Skantha had become anxious when the focus had been placed on him by the victim's mother.

But Eaton stressed no-one else in the room – including the undercover officer – had recalled such an exchange.

"Can I suggest to you, the reason you told police Lisa had brought up Vinny doing something to Amber was to make police focus their attention on Vinny," the lawyer said.

"I swear something like that was said," the witness told the court.

Shortly after the meeting, Skantha and Clinton dropped the teen at his home.

When he got out, he said, the defendant leaned out of the window and told him: "say anything and I'll kill you."

Clinton said she had not heard such a threat but the witness stressed it had happened.

Skantha had also earlier allegedly threatened to kill members of the boy's family and his cat.

The witness is expected to finish giving evidence later today.

Earlier today

The teenager – who has name suppression – was interviewed by police in the days following Amber-Rose's death on February 2 last year.

The witness said he had deliberately left a spot of blood on the defendant's shoe so officers had evidence to link him to the crime.

He said Skantha had asked him to clean the grey suede footwear when they were at his ex-girlfriend's house in Balclutha and they had been left in the garage of her home.

Defence counsel Jonathan Eaton asked how the teenager had cleaned them.

While he said he had done so with a damp rag he repeatedly said he could not remember whether he had used any cleaning product or what he had done with the rag.

In one interview with police, the witness said the clean-up had taken place Balclutha but days later, he said it had happened at Skantha's Fairfield home.

Today, he told the jury that had been a misunderstanding and that his first version was correct.

Eaton had a different take.

"What happened and the reason you're having so much difficulty remembering is that at Duxford [Cres], you must've put some blood on these shoes," he said.

The witness denied planting the evidence.

Eaton suggested the fact that Skantha did not mention the shoes after allegedly instructing the teen to clean them was implausible.

The defendant allegedly burned his bloodied clothes while in Balclutha.

The witness could not explain why the shoes were not burned too.

Eaton said he planted blood in Skantha's BMW as well.

"You must've had, whether on Amber's licence or something else, some of her blood. You had access to that and rubbed or flicked it around the car," he said.

"No," he replied.

The teenager previously told the court he drove Skantha to the victim's Corstorphine home, at his behest, and drove off a few minutes later when he hurriedly returned to the car.

Eaton suggested an alternative version of events.

"You didn't see Vinny come back to the car because he didn't leave the car did he?" the lawyer said.

"He did," the teenager replied.

"You get the key out from under the Buddha [statue on the doorstep] and went into her room?"

"No."

"Did you go and challenge Amber about what she was posting [on social media] about Vinny?"

"No."

"Did you get upset and do something stupid that you regret?"

"No."

While the defendant was allegedly inside the house committing the bloody crime, the witness made a phone call to a friend he had been socialising with earlier in the evening.

He said he did so because he was bored.

Eaton, however, suggested there was another motive.

"The reason you did that is because you wanted other people to know that at 11.54pm you were by yourself and Venod was not with you," he said.

"Did you do it to give yourself an alibi?'

The teen said "no".

Eaton said it was also significant he had asked his friend to put the phone on speaker mode so that more than one person would hear him.

The witness said he could not recall mentioning murder to them and if he had, it had been an "exaggeration".

While the witness hung up the call to friends at 11.55pm, telling them Skantha was returning to the vehicle, he now said that was a lie and he had actually done so because he wanted to listen to music.

Eaton said it was an example of the boy changing his story to fit the prosecution case that Amber-Rose was killed after that time.

"You're a little bit cleverer then you're playing at the moment and you've worked out if you saw Vinny coming back to the car ... he's back at the car by 11.55 and therefore could not have been in Amber's house attacking her after that period of time," Eaton said.

He put it to the teen that he had intentionally made the call while outside the vehicle, and possibly inside the Clermiston Ave house.

"No," the witness said.

The trial will continue tomorrow. Lawyers say they are on track to finish presenting evidence by the end of next week.

- additional reporting Amber Allott