A teenager who brutally bashed a sleeping mate to death, and has never explained why he did it, has today been jailed for at least 13-and-a-half years.

Heath Eric Morris, 19, admitted murdering 20-year-old Oliver Johnston, aka Oliver Greene on June 24 last year at Morris' family's Woodend Beach lifestyle block, 20km north of Christchurch.

Today, Morris was sentenced to life imprisonment with a minimum non-parole period of 13-and-a-half-years at the High Court in Christchurch.

Just why Morris murdered Johnston that day remains "unexplained at this point", Crown prosecutor Barnaby Hawes said.

Defence counsel James Rapley QC described the crime as "inexplicable".

"It's hard to understand how Heath finds himself in this position," Rapley said.

"His parents and younger brother are at a loss, struggling to comprehend what has happened to their son, brother, who lived a normal life and who they loved."

Morris had articulated "some remorse and insight" to a probation officer, Rapley said, and that he was sorry and wishes he could "rewind the night".

Justice Dunningham told Morris it appears he enjoyed a normal upbringing in a loving, stable family who must be as "perplexed and traumatised" by what he has done as everyone else.

The only danger signals in a pre-sentence report is Morris' prior binge drinking and experimentation with drugs, mainly cannabis, but twice with MDMA which ended in him being hospitalised. His parents say he showed paranoia after both episodes.

But the judge said the really troubling thing was that there is no "shred of an explanation" for what he did that night.

She described it as a brutal, unprovoked attack "on someone who thought you were his friend".

The court heard that Morris and Johnston met at a fast-food restaurant in Christchurch in early June 2018.

They began socialising and on Saturday, June 23 last year they'd arranged to meet at a party in a paddock near a forest in Webbs Rd, Amberley, organised by some local teens on Facebook.

The pair travelled to the party independently and got there about 7pm.

During the night, they socialised and drank together with a number of associates.

Between 7pm and 1am, Morris drank up to 12 bourbon and cola pre-mixed drinks, the summary of facts says.

Later on, Morris and one of his mates got into a fight with other partygoers and afterwards, Morris, Johnston, and another associate were driven back to Morris' house at Jelfs Rd, Woodend Beach, arriving about 1am.

During the short drive, Morris said to his associate seated beside him: "Shall we kill Oliver?"

According to the summary, he then said, "Bro, no one's gonna miss him".

He repeated the comment approximately five times during the journey.

Once they got home, they went into the main house and cooked and ate some food.

They then went into a small sleep-out 15m away from the Morris family home.

Morris and Johnston played video games on the Xbox while their associate went to sleep on a mattress nearby.

At some point during the night, Johnston has removed his shoes, got into a sleeping bag and fallen asleep on a mattress on the sleep-out floor.

Morris has grabbed a "striking implement with a 30mm round-shaped end and struck the victim multiple times to the left side of his head and face".

"The force of these blows was sufficient to break the victim's skull in multiple locations and break his jaw and cheekbone too," the court heard.

The injuries were "unsurvivable".

Morris removed Johnston's body from the sleep-out while the associate was still asleep.

He dragged him down a concrete path, over a gravel driveway, and through a gap in the fence into a small grass paddock.

Morris has then concealed Johnston with soil from an adjacent paddock, along with some long grass.

He's then gone back to the sleep-out and began cleaning the floor and mattresses with a spray disinfectant and cloth.

While he was hiding his tracks, the associate woke up and saw a large amount of blood on the mattress next to where he was sleeping.

When he asked what happened, Morris replied that he had "cracked Oli in the lip".

Asked why he'd done that, Morris said Johnston had been pestering him for a turn on the Xbox and that a friend from Christchurch had come and picked him up.

The pair woke again around 10am, the court heard, and Morris was further quizzed by his mate.

When asked if he had really hit Johnston, Morris said he had "cracked him twice".

Morris had breakfast with his family before returning to the sleep-out to continue cleaning up the blood on the mattress and on the floor under it.

Johnston suffered a broken skull, cheekbone, and upper jaw on the left side of his head, as well as a fracture from his eye to ear.

When spoken to police, Morris said: "I haven't done anything."

He declined to comment during a formal interview.

Johnston's family described him as a bright, multi-talented, fun-loving, articulate, adventurous, generous, friendly and caring young man with "a delightful smile and sense of humour".

"I believe that Oliver will be sadly missed by many people," one family member said in her victim impact statement today.

"The pain and suffering we have suffered in excruciating and ongoing. However, I do not want Oliver to ever be forgotten."

He was a talented musician, playing various instruments, as well as an enthusiastic outdoors man who had worked on the Spirit of Adventure and helped guide disabled skiers at Mt Hutt.

Johnston's murder was described by family as a cold blooded, brutal and callous murder, and a "tragic waste of young life".

They hope that Morris can be rehabilitated in prison and that one day, after "considerable time", be safely reintegrated into community.

Justice Rachel Dunningham praised the family for their poise and dignity and thanked them for bringing Johnston to life and giving "a true insight into what you've been through".