The body of Jarrod Leigh Turner, 22, was discovered by a passing motorist near the southern town of Richmond before sunrise on April 14 last year.

Shannon James Duffy, 31, has pleaded guilty to murdering his close friend.

Crown prosecutor Linda Mason told the Supreme Court of Tasmania on Monday the pair had been drinking together at a beach in the early hours of the morning.

Mr Turner posted a video on social media platform Snapchat showing him and his killer joking and laughing about 2.30am.

After leaving the beach, they pulled over near Richmond so Mr Turner could urinate.

As he did, Duffy retrieved a 12-gauge shotgun from under a car seat and shot him below his right ear, Ms Mason told the court, describing the killing as execution style.

"It is obvious from the footage that Mr Turner had no idea what was about to befall him," she said.

Ms Mason said Mr Turner was left for dead facedown on the side of the road.

"We all feel broken. Jarrod's death has torn apart our family. I try to hold them all up," his mother Michelle Bradley told the court.

Ms Mason said Duffy planned to kill Mr Turner after hearing he had made sexual advances to an underage girl.

He wrapped the shotgun in a yellow and black cloth before texting his one-time housemate to meet up.

The court heard Duffy destroyed evidence and avoided authorities for five days after the murder and initially pleaded not guilty before changing his plea in February to guilty.

In a November police interview, Duffy told officers he only meant to scare Mr Turner but he tripped and fell on the trigger, Ms Mason said.

Mr Turner's younger sister Lakeisha Pearce told the court she viewed him as a father figure.

"It plays on my mind constantly. My head hurts from thinking about it, my heart hurts from heartbreak," she said.

Several other people are facing criminal charges over the father of two's murder.

The matter has been adjourned to July 23 so Duffy's defence team can examine facts in the case.

Australian Associated Press