Patricia Page's son Michael "Billy" Hegedus was killed at his family's roadhouse at Inverell, in northeast NSW, on October 23, 1996.
"He was only 17, he hadn't lived," she said on Monday.
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 in the case 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.
But police, standing alongside Ms Page and stepfather Phil Page, on Monday launched a renewed public appeal, offering $1 million for information leading to the killer's conviction.
Recalling the evening of the murder, Ms Page said Billy, in his pyjamas, told his family good night before the shooting.
"He was sitting on the end of our bed when he was killed in the bedroom," she said.
"I can remember loading him into the ambulance and saying, 'Billy what the hell went on here?' and he said, 'I don't know, I don't know', he never told me."
Ms Page described the $36,000 - stolen from a safe - Â as not worth killing a 17-year-old over.
"They could have taken the money, they didn't need to kill him," she said, remembering her son as a boy who was "everybody's friend".
"He was cheeky, and he loved his sport and he loved his family."
Ms Page said she believed someone must have information about the unsolved murder as Inverell was a small town.
Similarly, Mr Page hoped that the boosted reward, 27 years after Billy's death, would bring the case closer to an end.
"It's an attractive figure of money and we just hope that it cracks someone," he said.
Homicide Squad commander Danny Doherty said police believed locals in the tight-knit area could provide a breakthrough, 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.
Police Minister Yasmin Catley said the reward would hopefully prompt people to cast their minds back to the time of the killing.
"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.
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