Shortly after Ayden left the party neighbours and guests at the gathering heard noises from the playground and went to investigate.
At the playground, witnesses saw three people standing over the victim who was on the ground injured, but the group quickly left the scene.
"Following the assault, the victim spent a week in hospital and is still recovering from the injuries sustained in the attack.
"This was an unprovoked, cowardly and vicious assault and we are determined to find the person responsible," Mr Leonard said.
The Frimley teenager sustained several injuries including a haematoma, fractured nose and chipped teeth, he needed stitches in his mouth and on his lip and his eyes were swollen shut.
While he was expected to make a full recovery, his mother Lisa Kairau said the past few weeks had been difficult and pleaded for anyone with information to come forward.
"He's not at school because he's too scared. He's going to be home for a couple more days before he goes back to school because he has assessments coming up but he is scared and I can't force him to go.
"It's emotionally and mentally draining us. It's too much. So just tell the police if you have any information."
She said her family couldn't move on from the assault, despite the fact Ayden was physically recovering well.
"I won't be moving on anytime soon. Especially if they catch him and we have to go through a court case." Police are seeking information about a further three boys believed to have been involved in the attack.
If you are one of these boys or know who they are, police ask that you come forward and contact Hastings police on 06 873 0500. Or call Crimestoppers anonymously on 0800 555 111.
"We know there will be people in the Hawke's Bay community who have the information we need to hold people accountable for this attack. Please don't hesitate to contact us if you can help," Mr Leonard said.