The blast occurred around 6:30am Friday morning in the upmarket Wynyard Quarter on the waterfront.

St John Ambulance has reported five people required hospital-level care, including four in a serious condition, and one first graded as critical but then re-classified as moderate.

A worker told news outlet Stuff the incident happened in a portable building at a construction site, suggesting a gas cooker had been left on overnight.

Frank Walsh, a senior fire officer at Auckland Central Station, told the NZ Herald they beat ambulances to the scene and attempted to assist the injured with fire hoses.

“On arrival we were confronted with five quite badly burnt patients,” he said.

“We then handed the patients over to the ambulance crews as more resources arrived.”

Burned clothing has been left at the site.