He said there were still 39 people in hospital, 11 critical and in intensive care. Seven had been discharged. One child, 4, had been transferred to specialist children's hospital Starship, in Auckland.
Four had died in ambulances on the way last night.
"Myself and my colleagues would like to pass on our heartfelt thoughts to the victims and their families," Robertson said. He said staff were doing their best, although some felt very angry.
"It's not something we would expect to happen in our country."
Robertson said the wounds from the gunshots were significant - some soft tissue injuries, some bones, some organs, some heads.
Many people would need ongoing surgery, he said, but the staff were capable of dealing with the trauma.
"Without a doubt it's unusual for surgeons in this part of the world to deal with gunshot wounds. We've had some experience while overseas… and we get some locally… but clearly we don't face the extreme load this put on our services at the time," he said.
"But we do have the skills."
He said pretty well all of the hospital's staff had been involved in responding to the huge influx of patients. At one point, 12 operating theatres were in action. Today and tomorrow there will be seven in use.
At some stages, surgeons were operating with armed police right next to them.
"It does put you on edge… but it gives you a feeling of protection, " he said.
The health board's chief executive, David Meates, said armed police were first deployed to the hospital after information suggested there was an active shooter there.
Later, when that proved to be incorrect, police stayed on, to protect families visiting loved ones, he said.
The death toll from the incident stands at 49. In the 2011 earthquake, 185 people died.
