"He had his hands by his sides when you hit him. To repeat, he was not confrontational in any way."
The punch was described and accepted as being delivered with considerable force. Coley had his hands by his side at the time.
It "clearly blindsided" Coley, the court heard.
"You ran from the scene and you told someone you know you had just 'king -hit' someone," Justice Davidson said.
Coley was rushed to hospital but passed away from his "unsurvivable" injuries. His cause of death was blunt force head injury resulting in acute subdural haemorrhage complicated by brain swelling.
Palmer, who is now 17, was caught by police and security staff in Don St soon after.
He admitted hitting Coley when first spoken with but initially said he acted in self-defence.
"You later changed the story and said the victim, Mr Coley, did not come at you, but you thought he was going to punch you, so you punched him in the head first. That is not an explanation which washes," Justice Davidson said.
Palmer's lawyer called for a sentence of home detention.
But Justice Davidson said he did not consider the purposes and principles of sentencing would have been sufficiently met by home detention.
The judge said he was "intensely troubled" by Palmer's use of drink and drugs at such a young age.
He said that combination must add to the risk of young people harming others, and themselves.
Justice Davidson sentenced Palmer to 22 months' imprisonment.