Two teenage boys who killed 19-year-old Billy Ray Waters have admitted they murdered him for "an ounce of weed".

Key points: Billy Ray Waters was bashed, stabbed and shot in the head

Billy Ray Waters was bashed, stabbed and shot in the head His body was found in bushland near the Mowbray Golf Club in August 2019

His body was found in bushland near the Mowbray Golf Club in August 2019 The two accused and Mr Waters had been friends "for some time", the prosecution said

William Adair Rothwell, 18, and Jacob Michael Brennan, 17, pleaded guilty to the murder in the Launceston Supreme Court on Friday.

Mr Waters was reported missing on August 11, 2019, having last been seen on Holbrook Street in Invermay.

At the time, police said a member of the public had called them on August 4 and reported hearing gunshots and a man screaming.

Police found Mr Waters' body later that month in bushland at Mayfield, near the Mowbray Golf Club.

The boys were 17 at the time of the murder and would usually not be named for legal reason, but Justice Robert Pearce made an exception due to the public interest in the case and the fact it was a "heinous crime."

Crown Prosecutor Daryl Coates said the two accused and Mr Waters had been friends "for some time", but Rothwell and Brennan became unhappy with Mr Waters as they believed he had stolen cannabis from them.

Mr Coates told the court the pair were given a sawn-off shotgun by another man, who told them they would "get paid an ounce of weed each to kill the deceased".

Mr Coates said on August 3, 2019, Mr Brennan sent a text to a friend saying: "I'm going to do one of the worst things in societies [sic] eyes tomorrow night… I'm worried the groups [sic] going to look at me differently."

The following night the trio met up at Vermont Road, Mowbray, where Brennan shot Mr Waters in the upper leg, before he and Rothwell repeatedly bashed the 19-year-old with the end of the shotgun, and stabbed him 18 times in the head, neck and back.

Mr Coates said Mr Waters was then shot in the back of the head, before his body was hidden in bushland.

"[The murder] was premeditated for some time… and it arose over very few reasons," he said.

"The murder was brutal and sustained and neither of the accused stopped the violence."

Mr Coates said a neighbour called police on the night of the murder and told detectives he had heard a gun shot, then a man screaming, "Someone help me, someone help me," before another shot was fired.

"It must have been horrific for the deceased," Mr Coates said.

The court heard both murderers had no prior convictions.

The boys were aged 17 at the time of the murder. ( ABC News: Laura Beavis )

'He was my rock': Grandmother and sister left heartbroken

In a victim impact statement read to the court, Mr Waters' grandmother Denise Waters said she had cared for her grandson for a number of years.

She said he was her "rock" and that he had recently started an automotive course in Devonport, in Tasmania's north west.

"When he was there he got mixed up with drugs … that was his downfall," she said.

"I miss not having him around. I've been having panic attacks ever since he died.

"No-one has the right to take someone else's life… [the accused] have destroyed not only Billy, but the rest of our family."

Mr Waters' younger sister Lillie told the court she had struggled ever since her brother's death.

"Not only does it hurt because he was my big brother, but he was my best friend," Lillie said.

"He was my hero. He was there for me for 16 years of my life.

"I had depression before this, but now I find it so much harder. I have massive trust issues… I don't have many friends now because I'm scared of what people will do.

"I hope one day the people who did this truly understand what they have done."

Rothwell's defence lawyer Evan Hughes said his client had struggled with night tremors since the murder.

The pair are yet to be sentenced.