The two teenagers charged with the Epsom motel murder in Auckland planned their assault and robbery while being driven around Auckland by the man who would later die from their attack, prosecutors allege.

The Crown this morning set out its case against 18-year-olds Beauen Wallace-Loretz and Leonard Nattrass-Berquist in the Auckland High Court.

David Johnstone introduced his prosecution argument by saying the two defendants had formed a plan to beat up and rob Ihaia Gillman-Harris, 54, and did so in a room of the Ascot Epsom Motel in the early hours of December 27, 2014.

But the defence lawyer representing Nattrass-Berquist told the jury the two teens were acting in self defence against an much older predator looking for sex.

The teens this morning both pleaded not guilty to the charge of murder and also pleaded not guilty to charges relating to the theft of cash, a cell phone and a bank card, as well as a charge related to the taking of Gillman-Harris' $15,000 Range Rover.

Crown prosecutor David Johnstone told the jury that Wallace-Loretz and Nattrass-Berquist had had "previous dealings" with Gillman-Harris and that Gillman-Harris picked up the two teens that night in Pakuranga, as well as another person, and drove them through the city.

Mr Johnstone said while driving around Auckland, the teens - then 17 years old - formed their plan via text message while sitting in the back seat behind Gillman-Harris, who was driving.

Part of the text exchange between the teens included the question: "G, should we roll him?"

Mr Johnstone said some of the other text messages during this exchange suggested the severity of the assault that was going to take place, including the question: "Hospital?" And the answer: "Yeah G, all day".

He said Gillman-Harris and the two teens later arrived at the Ascot Epsom Motel, but dropped off the third passenger before doing so.

Before arriving, Gillman-Harris also drew cash out.

It was between 8.17am and 8.30am that morning that the Crown said the crime took place.

It was during that 13 minute period that Mr Johnstone said all three parties were inside the motel room.

Wallace-Loretz and Nattrass-Berquist left the room at 8.30am and drove off using the victim's Range Rover, he said.

Inside the room, Gillman-Harris had been left with severe bruising on his left arm and hand, as well as his torso.

The "long and straight" bruise patterns suggested what type of weapon was used, Mr Johnstone said.

But more serious was the multiple skull fractures and brain trauma the victim received as a result of the attack.

Gillman-Harris did not immediately die and was able to leave the motel room and attract the attention of motel staff a short time later.

He was taken to Auckland City Hospital but died later that day.

Mr Johnstone said that the weapon used was a bat or similar object, which the two teens brought with them to the motel, used, and then took away from the scene.

The jury also heard about a computer that was found smashed in the motel room afterwards.

Mr Johnstone said a computer expert, having later looked at the hard drive, found that the computer had been switched on and immediately connected to the internet.

Before it was smashed, a video from a pornographic website was briefly screened.

But defence lawyer Murray Gibson, representing Leonard Nattrass-Berquist, argued there was "nothing straight forward" about the case and that the two teens were acting in self defence that morning.

He said Gillman-Harris was a predator who had an ulterior motive with the teens and that he had planned on having a "sexual encounter" with one or both of them.

Mr Gibson said Gillman-Harris supplied the teens with alcohol while driving around Auckland.

After later checking them into the motel, telling staff the teens were his nephews, Mr Gibson said Gillman-Harris switched on a computer and a porn video.

But Nattrass-Berquist was not interested and slammed the computer closed.

It was at this time, Mr Gibson told the jury, that Gillman-Harris became angry.

He then sexually assaulted Nattrass-Berquist, Mr Gibson alleged.

He said during this time Wallace-Loretz was in the bathroom, and came out to find his friend being sexually assaulted.

Mr Gibson said the two teens then acted in self defence against Gillman-Harris and a fight ensued.

He said they did not aim to kill him.

Defence lawyer John Kovacevich, representing Beauen Wallace-Loretz, did not set out an introduction to his defence this afternoon.

The jury for the four-week trial consists of three men and nine women.

The trial is before Justice Kit Toogood.

A four week trial was originally set for November last year, but was adjourned until April 4.