"He was jerking and thrashing. They waited until the seizure finished and brought the stretcher in to take him out. He looked like he was only 18."

That was the horrified reaction of one woman at a dance concert marked by apparently drug-fuelled incidents and the hospitalisation of five people - three of whom were rushed to Auckland Hospital in a critical condition after taking drugs.

Several concert-goers told the Herald On Sunday that many people appeared to be drugged. Several partygoers climbed the 25m-high marquee set up for the Listen In event at Mt Smart.

"You knew everyone was on drugs," the woman who witnessed the seizure said.

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"People were doing really weird dance moves. Everyone was either really friendly and touching or really pushy."

She also saw people running on the roof of the tent.

A party-goer climbs the marquee.
A party-goer climbs the marquee.

The concert drew 20,000 people to what was sold as "the largest marquee in Australasia" erected on Mt Smart's field, to watch high-profile international acts Flume, Diplo and ScHoolboy Q.

Another festival-goer described the wild scenes like nothing they had ever witnessed.

"It was actually quite insane. Towards the end of the night you could really see the impact of whatever they had taken.

"I remember looking down and there was literally a girl being wheeled out of the concert unconscious. Her head was back, her mouth was open, body was limp.

"Then I saw a chick being walked out of the mosh pit and she was fully chewing her lips. She was on something. She was chewing her lips like bloody nobody's business."

A party-goer climbs the 25m-high marquee at the event where five people were hospitalised. Photo / Supplied
A party-goer climbs the 25m-high marquee at the event where five people were hospitalised. Photo / Supplied

She stayed after the concert to look for jewellery she had lost.

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"There were so many of the little bags that you put drugs in, they were littered all over the ground," she said.

This comes six weeks after reports of mega-dose party drugs arriving in New Zealand which have been linked to deaths overseas.

The new drugs have names like "The Punisher", "Putin" and "Trump". The Punisher pill has been tested as containing 330mg of MDMA – nearly five times a normal Ecstasy pill.

It has not been confirmed whether mega-dose drugs were being taken at the event.

However, footage has emerged of numerous concert-goers climbing on top of the tent, which had been flown in from Australia.

Within the tent itself, audience members can be seen climbing the metal support pillars, many metres above the crowd.

Revellers climbed the largest marquee ever to be erected in New Zealand. Photo / Supplied
Revellers climbed the largest marquee ever to be erected in New Zealand. Photo / Supplied

Violence was also an issue.

"There were so many fights that broke out," the witness said.

Security were checking some people for drugs and alcohol at the gate.

Auckland Stadiums director James Parkinson, who oversees Mt Smart Stadium, said security checked bags of concert-goers and patted down "profiled individuals".

"We had safe disposal units for drugs at all gates and a volume of drugs were confiscated from patrons at the gates. Both security staff and police were monitoring the site for drug use throughout the event."

Parkinson said they considered inviting Know Your Stuff NZ - a group who test drugs at festivals - to the event.

"However, these drugs are illegal substances and are prohibited items in our venues. The concept of testing and returning illegal substances to patrons places us, as a venue operator, in a very difficult legal position."

Parkinson denied there was widespread drug-taking.

"We had very good security measures in place, which meant the vast majority of the over 20,000 crowd had a great night.

"Unfortunately, these incidents do highlight the inherent risks of drug-taking."

On Saturday evening, Auckland District Health Board said one of the three critical patients remained in critical care but in a stable condition, while the other two had been discharged.

Police were at the event, but no major issues were reported, a spokeswoman said. They were not immediately able to say whether inquiries would be made to attempt to identify the drugs involved in the hospitalisations.