"It's a bit challenging for people ... we are struggling with it as much as anybody else," Robertson said.
"This is not something we expect to see in our environment ... we see gunshot wounds, but 40 to 50 in one day is not what we expect."
"Dealing with 50 people in a day is more than what we should see, so our staff are doing remarkably well."
Most people cope pretty well but it's when they go home that they start to think about it, he said.
Staff were "horrified, stunned and angered", he said.
He spoke about the impact of having gone through the Christchurch earthquakes, and how the experienced staff have dealt with trauma.
"We have been there."
"We only had one death in a patient that arrived at the hospital. Those who got there have a chance. Those who didn't get here didn't have a chance."