Students "preloading" on alcohol before the event and smuggling booze into the venue have been blamed for the incident.

Police have launched an investigation into a boozy school ball which left nearly 50 students needing medical attention – including two ending up in hospital.

Saturday night's unofficial Wellington College ball finished with 37 students needing treatment for alcohol-related complaints, and a further 11 treated for injuries.

Students "preloading" on alcohol before the event and smuggling booze into the venue have been blamed for the incident.

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It is understood 700-1000 students were at the event, which was held at the Wellington City Council-owned Shed 6 on the waterfront.

Police say they are now investigating, with council staff working alongside officers to find out how so many students became so intoxicated.

Wellington College principal Roger Moses said he believed the ball was organised by a group of students whose parents had paid the bond for the venue.

"The school has no control over a private function organised by parents. If parents want to arrange a function, then they are quite within their rights to do so."

Moses said he knew who those parents were.

"I am aware of them, at this stage I have not contacted them," he said.

There had not been an official Wellington College ball for over twenty years, due to the fact they were often ruined by intoxicated people.

"In my very determined view, at best nothing goes wrong, at worst they can be a disaster," he said.

Council spokesperson Richard MacLean said they were aware of the incident and were working with police to find out what happened.

The venue had supplied 20 security guards to monitor the event.

"There were problems with preloaded people turning up, and problems with people smuggling things in," he said.

MacLean said they would now be reviewing their security arrangements.

The event was not issued with a liquor licence, he said.

"Police are taking this seriously, and so are we."

Moses said it was entirely parents' responsibility to care for their children outside of school hours.

"The school has no legal responsibility over what a student, beyond school activities, does. This is a parental issue."

A police spokesperson said they were still gathering evidence in regards to Saturday night's event, and would decide on an appropriate course of action once all the facts were known.

Police said the ball highlighted the issue of excessive drinking.

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