Connolly said there was about 150 to 200 young people at the party, that was "relatively well behaved".
However, police were called to control the crowd while the Fire Service, St John and Auckland Westpac Rescue Helicopter treated Hemi.
"Hemi was pretty much unresponsive, he was status one. It was mainly head injuries, that was our main concern."
As well as treating the teen, who is a keen rugby player for the Otamatea Hawks, Connolly was also liaising with Hemi's mother.
"His sister and cousin were there and we were liaising with his uncle and mum. It's not the first time I have arrived at an incident to find my family in it.
"It's never easy whether it's your family or people you don't even know, it's just the work you have to do - get stuck in and do the job and give them the best chance you can."
Hemi, who is from Kaiwaka, was flown to Auckland Hospital about 1.30am, still in a critical condition, said Connolly.
An Auckland District Health Board spokeswoman said he was in a stable condition today.
Otamatea Hawks treasurer Jennie Reynolds said Hemi is well known in the local community.
"He is a great support for the rugby boys, he is always at rugby and always shows up and is always helping as he has grown older.
"He is a really great support for us."