"When I rocked up there were some people passed out. I was out to it."
He also said "hundreds were really drunk".
He believed some students snuck in alcohol in their pants.
Wellington City Council spokesman Richard MacLean said the venue was hired without a liquor licence.
"There was never any intent liquor would be served or drunk," he said.
About 18 security guards were monitoring the event.
Mr MacLean said he was aware a few students were "passed out" and an ambulance was there to treat and take them to hospital if necessary.
Mr Moses said the school would not be changing its stance against balls any time soon.
"I will absolutely not change my mind. Functions like this can turn to custard."
Mr Moses said it was a private function and the school had nothing to do with the organisation of the event.
"This function had no school input. We did not advertise it. We didn't even know it was on."
He said he did not have control over what students did outside of school.
"What people don't understand is that school does not have control over students outside of school hours. It's a parent issue, not the school's."
He said the matter was being dealt with by police and he would not be speaking to students until police had completed their investigation.
Wellington College has not held an official ball for 20 years due to problems with intoxicated students.
The unofficial ball was held at council-owned Shed 6 on the Wellington waterfront on Saturday night. About 800 people turned up to the event.
A Wellington police spokeswoman said officers were called to an event at Shed 6 on Queens Wharf.
"Forty-eight young people attending the event required medical treatment for alcohol-related illness or injuries," she said.
Of those 48 students, 37 were understood to be intoxicated and a further 11 needed attention for injuries.
The police spokeswoman said officers continued to monitor the event throughout the night.
"This event highlights the necessity for people to drink responsibly. Excessive alcohol consumption lowers a person's inhibitions and ability to make good decisions.
"While police want people to have a good time we remind everyone to drink responsibly and keep themselves and their friends safe."
She said police were still "gathering information" about the event and would "decide on an appropriate course of action once all the facts were known".
Shed 6's management will be further reviewing its practices to see how the risk of a repeat incident can be minimised.
Glenys Coughlan - chief executive at Positively Wellington Venues, which is responsible for Shed 6 - said the numbers of those intoxicated and needing medical attention were higher than usual.
But she said the company had undertaken a comprehensive risk assessment prior to the event. Several months prior it had already been looking at a number of other improvements, including bringing in breathalysers and ensuring police were kept in the loop around such events.
"We had several meetings with the organisers and the parents where the Health and Safety was outlined very clearly."
As a result of this extra security was on hand, there was a minimum food spend required and Wellington Free Ambulance were on site.
However, she said despite the higher levels of security a number of students were still able to sneak alcohol in through small Ribena drink packets that are soft and difficult to detect when hidden well.
While Ms Coughlan acknowledged there was a higher number of intoxicated students - the roughly 750 others at the event were believed to have behaved responsibly and had a good time.
She said the company would be meeting with parents and police shortly to "debrief".
