The Castle St courtyard on Saturday after the deck collapsed.

A witness has described scenes of "carnage" as a balcony collapsed, leaving two people seriously injured, at a surprise gig by Six60 in Dunedin.

Police were trying to get people off balconies and roofs about 7.45pm on Friday when a balcony occupied by up to 20 people fell in Castle St, in North Dunedin.

The collapse threw people to the ground from a height of some 3 metres and "collapsed decking" landed on people in the courtyard area.

Hamish McNeilly / Fairfax There were multiple injuries after a balcony collapsed at Six60 gig in Dunedin.

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﻿Eighteen people were taken to the emergency department at Dunedin Hospital.

RILEY BAKER/SUPPLIED Eighteen people were injured in the balcony fall.

A Southern District Health Board spokeswoman said one patient had been transferred to Christchurch Hospital and another remains in a serious but stable condition at Dunedin Hospital.

The remaining 16 patients have been discharged.

Police said up to 1500 people attended the concert being held in a courtyard space between private rented accommodation. Several people climbed onto roofs and some were on balconies overlooking the concert venue.

RILEY BAKER The number of people on one of the balconies fluctuated from about nine to 18 or so at the point of the collapse.

Tahnae Wilson-Brown was lucky to avoid injury.

"I was under the balcony five minutes before it happened," he said.

As Six60 took to the stage, excitement built all around, and Wilson-Brown decided to move forward, to get closer to the band.

RILEY BAKER/SUPPLIED Police and bystanders rush to help after the balcony collapsed

"We just heard a big crack and turned around and the bloody balcony had collapsed."

He said "carnage" ensued and many people were stuck under the balcony.

A group of students and police lifted the balcony off the trapped people. Several people were crying and at least one badly injured person was screaming.

RILEY BAKER/SUPPLIED The balcony fell about 3 metres.

The balcony was mostly intact, he said, but some fragments had splintered off.

Wilson-Brown, in his fourth year at Otago, said the event was unprecedented and a "terrible thing to see happen".

He said excited people jumping on the crowded balcony possibly caused its collapse, which happened in a matter of seconds.

RILEY BAKER/SUPPLIED Up to 1500 people crammed into the surprise Six60 concert in Dunedin.

Wilson-Brown said some people in the crowd continued as if nothing had happened, and wanted Six60 to keep playing.

For others, especially those close to the injured people, it was a "sobering" incident, he said.

Wilson-Brown said he expected some students to be on edge for a while but the city's party culture would probably return to normal within a few weeks.

RILEY BAKER/SUPPLIED Emergency services and members of the public carry an injured person to an ambulance after a balcony collapsed near a surprise Six60 concert in Dunedin on Friday.

He said Six60 "did a good job" of keeping the crowd relatively calm.

Police said their officers and Campus Watch were trying to clear people from the roofs and reduce the number of people on balconies when one of the balconies collapsed.

Six60 bass player Chris Mac said on Twitter the band played the show "on the advice of security and police who we were working with closely".

ANTHONY CHILCOTT-PARKER Up to 20 people were on the balcony when it collapsed at a surprise Six60 gig in Dunedin.

The band also posted on its Twitter page that it was "very upset" that people were hurt.

"Of course we feel responsibility. We feel awful."

University of Otago campus cop Constable John Woodhouse said the balcony was about 30 metres from the stage where Six60 were playing.

HAMISH MCNEILLY/FAIRFAX NZ Eight people are seriously injured after a balcony collapsed during a Six60 concert on Dunedin's Castle St.

"We were busy removing students from roofs and from balconies before it happened, but unfortunately we couldn't get access to that balcony."

Officers had been concentrating on balconies closer to the stage when the collapse happened.

"The noise of the crash was drowned out by the music."

SUPPLIED Six60 outside 660 Castle St in Dunedin.

Six60 kept playing until about 8.40pm, on the advice of police, to "avoid a panic", Woodhouse said.

"There was a big surge when it happened, everyone wanted to move forward. Within 30 seconds everyone knew what had happened."

He arrested one student who refused to get off a roof.

One witness said "everyone went crazy" as the music started.

"I think a lot of people were jumping on [the balcony]. The whole thing started cracking and it all just fell down," he said.

His friend said he heard a "cracking noise" and initially thought someone was throwing a bottle.

The pair knew a couple of people who were on the balcony when it collapsed, but they were "all right", they said.

'WALKING WOUNDED' AMONG THE INJURED

Police southern communications shift commander John Doherty said aside from those taken to hospital, "walking wounded" were amongst the crowd of students.

HAMISH MCNEILLY/FAIRFAX NZ Emergency services at the March concert struggled to get through the crowds, and some of those injured had to be transported to hospital in police cars.

"There are several patients with broken bones."

Emergency services struggled to move injured people away from the street due to the number of people at the concert.

St John spokesman Ian Henderson said patients had a range of serious injuries, including possible head injuries and fractures.

It was not known whether all of the injured were students or not, Henderson said.

Henderson said St John sent four ambulances and a rapid response vehicle to the scene initially.

A group of students were seen picking up a van and moving it to make way for emergency services at one stage.

​Six60 formed in 2008 at their flat at 660 Castle St, which inspired their name.

The surprise gig on the infamous party street began with rap artist Arcee.

University of Otago Vice-Chancellor Harlene Hayne visited the scene on Friday night and said she was "deeply concerned" and would be seeking as much information as possible.

"Students were visibly concerned for the welfare of their fellow students and many provided valuable assistance to the emergency responders," Professor Hayne said in a statement.

"This activity was not on University grounds," she added.

"However, once the University confirmed late this afternoon that the event was to take place at a private property in North Dunedin, its own security staff, Campus Watch, attended along with the Police as a precaution and fortunately were present to help out with first aid to the injured immediately after the balcony collapsed."

Otago University Students' Association president Laura Harris said the news of the balcony collapse was upsetting.

"Our focus is on supporting . . . any students who have been affected and we urge anyone requiring support to contact help@ousa.org.nz."

Police said Worksafe New Zealand would also investigate the balcony collapse.

SECOND INJURY WOE

The band's gig in Dunedin in February last year was also marred by injury.

One man suffered head injuries after trying to climb a roof to get a better view of the backyard set.

Campus Watch members and police were present at that event, and repeatedly told onlookers to climb down from precarious positions.

Band member Marlon Gerbes later contacted the 20-year-old, who was discharged from Dunedin Hospital with a concussion and stitches.