But he said it was as safe as any other workplace he had laboured at.
"It could have been any one of us," another worker said. "It's horrible."
Staff from the site said they had been sent home for the day.
Three WorkSafe NZ investigators are at the scene.
A spokeswoman for Progressive Enterprises, the company Countdown is part of, provided a brief statement.
"We were very concerned to hear about an incident at the Ponsonby site today.
"We take health and safety seriously and are discussing this incident with the site development contractor."
Ron Macdonald, general manager, northern region, for Ebert Construction said the incident was under investigation and they were cooperating with Worksafe.
"Our thoughts and prayers are with the injured man and his family at this time."