The man behind the trigger was high school acquaintance Matthew Perry, 23.

On Wednesday, Perry stood before the NSW Supreme Court and pleaded guilty to murder - eerily on the eve of another State of Origin game.

He stood wearing a grey suit and shirt as he was formally convicted by acting Justice Jane Mathews.

Mr Coates' mother, Julie Lowe, left the court with family after hearing the guilty plea but did not make a comment.

Mr Coates had been at the Colyton Hotel watching the State of Origin game on June 18,  just weeks after celebrating his 18th birthday, when police said he became involved in a dispute with Perry.

A police statement tendered previously to Mount Druitt Local Court said the victim was kicked out of the Colyton Hotel earlier in the night and walked less than two kilometres to Nevada Avenue.

In this street, another fight broke out between the pair. The victim armed himself with a shovel and the accused with a rifle.

Neighbours reported hearing an argument and loud noises in the street just before Mr Coates was gunned down not far from his home.

One neighbour mistook the noise for fireworks let off by fans celebrating the Blues' Origin win.

Police and paramedics arrived a short time later to find Mr Coates suffering from a single gunshot wound.

He was taken to Westmead Hospital but died just before midnight, less than an hour later.

Mr Coates' twin brother, Sam, paid tribute to his sibling online shortly after the fatal shooting.

"Rip Jed my brother my twin your a king in my eyes love you bro."

Other friends paid tribute to Mr Coates on social media.

"Rip Jed ... my brother. I love you brother I carnt [sic] even get my head around this," wrote one friend on Facebook.

"You will always be in my heart ... your legacy will always live on, im devastated we couldn't go out together and drink you were taken way to quick and young."

Perry has been in custody since his arrest two years ago.

He is expected to appear in court on Friday for his sentencing hearing.