Auckland Stadiums director James Parkinson, who oversees Mt Smart Stadium, said they recorded five ambulance trips from the stadium last night and four of them were drug-related.
"We had very good security measures in place, which meant the vast majority of the over 20,000 crowd had a great night," Parkinson said.
"Unfortunately, these incidents do highlight the inherent risks of drug taking."
One concert-goer posted on Facebook this morning that they "saw someone being wheeled out not even conscious and watched multiple girls almost chew their lips off".
Dr Jez Weston, from drug testing and harm prevention advocacy group Know Your Stuff NZ, said they did not have a presence at the Listen In event.
However, Weston said 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."
Australasia's biggest marquee was flown in for the Listen In concert which featured Flume, Diplo, ScHoolboy Q, 6LACK, Slowthai and others.
James Peterson, of Regional Facilities Auckland, which operates the stadium, said the original plan was to install a smaller tent on a carpark for a crowd of up to 15,000, but a sellout crowd of 20,000 required a shift to the stadium floor and a bigger tent.
