The ruling meant the case against Mercury was doomed, prosecutors concluded, and Mercury walked free.
Mercury moved to Melbourne in the 1970s, where he is believed still to live.
Buckingham read out the boy’s name and his confession to the police.
“I was down the beach that day and I saw a man drive past me in a car with a little girl,” Mercury told police.
“I walked along the beach to Wollongong and I saw the man driving back on his own.”
“I am worried, I did that to the little girl, I didn’t mean to do it,” Mercury said seconds later.
“Do you mean that you had something to do with the disappearance of the child, Cheryl Gene Grimmer?” police asked.
“Yes,” Mercury said.
Mercury went on to describe the girl being lifted up to drink from a water fountain before other children left her alone.
Mercury said he knew a man was nearby, so he put his hand over the girl’s mouth as he grabbed her to keep her quiet.
“I took her by the hand and put one hand around her mouth and carried her around to the sand hills,” he said.
“I then continued up to Bulli Pass, where I took the little girl.
“She started to scream when I got her up there. She would not be quiet. So I put my arms around her throat and strangled her.
“I left her lying on the ground at the side of a tree. I covered her up with bushes and leaves and threw some dirt on top.”
Buckingham became emotional while reading details from the confession – pausing and breathing deeply to regain composure.
Buckingham will address the media with one of Grimmer’s brothers later on Thursday.
More to come
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