Michael "Billy" Hegedus, 17, died from wounds after being shot twice at his family's roadhouse at Inverell in northeast NSW on October 23, 1996.
Around $36,000 was stolen from the service station in what police believe was a robbery gone wrong.
A $100,000 reward for information leading to a conviction for Billy's death was offered in 1997 and the following year Justin McInerney was charged over the killing.
But after two aborted trials and a hung jury in a third, Mr McInerney was acquitted of murder in a final trial and walked free in 2001.
Following the fourth trial, police said the case was closed and would remain that way unless fresh and compelling evidence emerged.
However, 27 years after the killing police have launched a renewed appeal for information.
Billy's mother Patricia Page and stepfather Phil Page joined police in a public appeal at Armidale on Monday to help solve the murder.
Recalling the day of the murder, Ms Page described Billy as "everybody's friend" and a beloved member of the local community.
"I remember waking him up that day, hassling him to get on with studying for his HSC, but he just fluttered his big, beautiful eyes at me and told me not to worry," she said.
"In our hearts there's not a day that goes by we don't ask, 'what if?' or 'what happened?' As a parent, you just never stop looking for answers."
Ms Page said Inverell is such a small town, that she truly believes someone must know something about that night.
"And if you know something, please say something to help find justice for our Billy," she said.
Police Minister Yasmin Catley said the reward would hopefully prompt people to cast their minds back and reconsider any information that could help detectives.
"No piece of information is too small and something you might have previously disregarded may just be the missing piece in this puzzle," she said.
Homicide Squad commander Danny Doherty said police believed someone in the tight-knit local area could help crack the case, calling the murder "reckless and callous".
"Billy was just a kid who loved his sport and had dreams of becoming a teacher ... he didn't deserve what happened to him, his family deserve answers," he said.
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