One person seriously injured after a balcony collapsed in Dunedin has been transferred to Christchurch Hospital.
The collapse happened on Castle Street about 7.45pm last night, where about 1500 people had gathered to watch New Zealand band Six60.
Eighteen people were taken to Dunedin Hospital, two in a serious but stable condition. One has since been transferred to Christchurch Hospital.
The remaining 16 people have been discharged.
Police had been trying to clear people from roofs and balconies of apartments near where the concert was happening when one of the balconies collapsed, throwing people to the ground from a height of about 3m.
Witnesses said people were crammed three-deep onto the balconies and were jumping up and down on them.
Police said they had been warning people to stop jumping before the balcony and decking collapsed on people in the courtyard below them.
Police spokesman Inspector John Doherty said the concert was in progress when it happened.
"As a result of that a number of people were injured, some with serious injuries and they have been transported to hospital by ambulance and police."
Very upset that people were hurt tonight. Massive thanks to the local police and security who were working with us to help keep people safe.— SIX60 (@SIX60) March 4, 2016
Inspector Doherty said there had been some difficulty getting people out initially because the area was congested due to the concert going on and the number of people in attendance.
Jennifer Bell was close to where the first-floor balcony collapsed and said people couldn't move, even if they'd wanted to.
"So that's what would have been scary is that even when people knew the balcony was coming down they couldn't have got out the way in time because it was packed. Even if they tried they would have been stuck under there."
University student Morgan Oakley was at the concert, and said at least 300 people were in the courtyard.
He saw looks of shock on the faces of the Six60 band members when it collapsed, he said.
People there told him that about 20 concert-goers were jumping up and down on the small balcony, which was really only made to hold five people.
Another witness, Matt Bradley, said his group had to push through the crowds to get to their position.
"And then afterwards people were getting kicked out, they weren't letting anyone else in," he said.
"We were about this area. There was maybe the only space in front of you. Everyone behind us was people shoulder to shoulder, stomach to stomach, on roofs, trees. That tree had about 10 to 20 people in it."
"If you think of it like this, Six60 sell out the town hall with 3000 people and that's 3000 paying people. Now imagine they're coming down to the student quarter, everyone's going to turn up."
"Yeah we were all here. Basically we turned up and it just fell. It just started, like, cracking then it fell down. By the time we turned around after we heard the cracking it had fallen down.
Another witness said, "you could see it bouncing, yeah and the balcony was just bouncing up and down. You heard it crack and just saw the end of it.
Police had told them to stop but they didn't and it collapsed, falling 4m on to the bitumen below, he said.
A large number of police officers were still in the courtyard at 9.30pm about two hours after the collapse, and clean-up from the concert was ongoing.
The Otago University Students' Association President Laura Harris said: "We are saddened by the news of the balcony collapse earlier this evening ... Our focus is on supporting any students who have been affected."
Ms Harris urged anyone requiring support to contact firstname.lastname@example.org.
Six60, who formed in 2008 at 660 Castle St, usually performed there.
That flat's property manager Stef Ward said she had been told the backyard was not big enough, so the band had moved the concert to 598 Castle St, a complex of nine apartments.
Ms Ward was at the courtyard earlier today when the band was setting up, and said the balconies had appeared solid in what was a fairly modern complex.
Mr Bradley said the band had stopped playing when the balcony fell.
"Six60 actually made sure everyone was all right before they continued. They kicked everyone off the balconies, because, they're not here to see people get injured they're here to make sure everyone has a good time."
The band's bassist, Chris Mac, said on Twitter that the band had been told to keep playing for safety reasons after the balcony collapsed.
Hope everyone from the balcony is ok. Was an amazing time outside if that.— CHRIS MAC (@chrismacattack) March 4, 2016
University of Otago vice chancellor Harlene Hayne said she was deeply concerned by the incident.
"Our thoughts and deepest concerns are with the students, their families and friends at this time," she said.
"This activity was not on university grounds. The first that university staff heard of this surprise concert was via social media and the university emphasises this was not an official event of any sort. The bands involved organised the event.
"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."
Ms Hayne visited the scene last night and said she would give her full support to the affected students and families.
She said she will seek as much information as possible about the gig.
Worksafe New Zealand will investigate what happened over the next couple of days.