Scenes of drug-fueled "insanity" erupted at a dance concert last night with party-goers climbing the 25m high marquee set up for the event and climbing scaffolding.

Five people who attended the Listen Inn event at Mt Smart were hospitalised, three rushed to Auckland Hospital in a critical condition after taking drugs. One person who had taken drugs was in a moderate condition, and a fifth person's injuries were unrelated to drugs.

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.

One festival-goer described the wild scenes as 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 like 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."

She stayed at the venue after the concert had finished to look for some jewellery.

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

"You could definitely see there was quite a big drug presence there, just by all the bags that were laying on the ground, the amount of girls that did not look drunk, they looked high, or on something.

"Especially the girl who was chewing her lip, I was so worried."

Footage of numerous concert goers climbing on top of the tent, which had been flown from Australia, has emerged.

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

Violence was also an issue at the event.

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

"Security guards were pulling girls over the fence just to get them out because it was that dangerous.

"Me and my two other friends were actually pushed to the ground. We were literally trampled on.

The largest stadium tent in Australasia is sitting in the middle of Mt. Smart Stadium to host the Listen In dance concert this Friday night. Video / Cameron Pitney

"I go to festivals quite often and that was by far the craziest I've been to. In the mosh pit no one cared. If you were hurt or anything they would just trample all over you. It was ridiculous, insanity."

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."

Consideration was given Know Your Stuff NZ - 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."

He said they recorded five ambulance trips from the stadium and four of them were drug-related.

However 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."

This morning, Auckland Hospital said the three critical patients were stable.

St John Ambulance head of public affairs Victoria Hawkins said earlier they could not confirm if the people hospitalised had a drug overdose.

"It was due to a substance they had ingested. I think our people at the event were concerned."

Dr Jez Weston, from Know Your Stuff NZ, said that at events they were invited to, people who were informed they had a different substance to what they thought they had generally discarded it.

"This is a tragedy and we're hoping for the best for everybody involved," Weston said.

"We were not at that event so we don't know what those people might have taken. Those people themselves probably don't know what they actually took, the medics there don't know what they took, so it's harder for the medics to respond.

"This year we've been seeing people take too much MDMA, cathinones and people taking unknown and new substances.

"Fundamentally, young people shouldn't have to suffer from a night out and that's what we're trying to achieve here.

"Where we go depends on the organisers and whether the organisers are willing to take the legal risk to have us at their event. Because we operate in a legal grey area."