Five people were taken to hospital after a gas explosion at a building site in Auckland's Wynyard Quarter this morning.
Four people are being treated in the burns unit in Middlemore Hospital - three are critical and one is stable.
A fifth person is in a moderate condition in Auckland Hospital.
The incident was reported to police about 6.25am in Halsey Street in Auckland's CBD.
National Commander Russell Wood told Midday Report that firefighters at the scene gave first aid to the injured and made the situation safe.
He said the explosion was now the subject of an investigation by WorkSafe and possibly the police so he was unable to make further comment.
It is believed an LPG bottle exploded at the building site. Bits of burnt clothing could be seen scattered around.
The building site is in a very busy part of downtown Auckland.
Hawkins Construction declined to comment.
Parent company Downer confirmed in a statement that an incident had occurred at the Hawkins construction site in Wynyard Quarter.
It said it was working with authorities to establish exactly what had happened and its immediate focus was on supporting the injured workers, their whānau, and the team on site.
"We wish to acknowledge the efforts of the first responders - Fire & Emergency New Zealand, the New Zealand Police and St John New Zealand. We are a tight team and our thoughts are with the injured workers and their whānau."
Downer said it would not comment further until an investigation into the explosion was completed.
WorkSafe has opened an investigation into the incident.
Multiple ambulances and fire trucks were at the scene this morning.
Fire investigators remain on site. They say it could be days before the cause of the explosion is determined.
A St John Ambulance spokesperson has confirmed it was an explosion.
It said it sent 10 vehicles, including five ambulances, to the scene.
It's believed a gas bottle was the cause of the explosion but that has not been officially confirmed.
Master Plumbers chief executive Greg Wallace said it was common for construction workers to use portable gas cookers.
"You can count on one hand across New Zealand the amount of gas issues we have like this. It's pretty clear to me that it's a portable device that's just used in a temporary situation."
Such cookers did not have inbuilt safety mechanisms and should be used with caution.
"Things like camping devices and barbeques don't have those so that means there is greater risk, particualrly if the gas is left on overnight."
It was not confirmed exactly what caused the explosion but Wallace said if it were an LPG bottle, it was important to know 9 kilogram bottles could not be used indoors.
"Barbeques and other gas hobs, you should always be getting a licensed gas fitter to do the installation because ventillation and correct installation is so critical for safety"
Gas has a strong smell but if it settled it could go undetected.
"Sometimes due to the atmosphere or the size of the room the gas can settle below head level and so sometimes this means, without proper ventillation, people don't automatically smell the gas when they come in."
It was also important to install gas detectors when cooking with gas indoors, Wallace said.
Council of Trade Unions president Richard Wagstaff said he wanted to see a full investigation into the explosion at the building site in Auckland.
The health and safety system at the building site had not kept workers safe and needed to be put right, Wagstaff said.
"Today's explosion could very well have proved fatal so we are very lucky that no one was killed," he said
"It's important there be a full and detailed investigation into how this happened and steps are taken to better protect workers."
Every New Zealander deserved to know they would be safe at work, Wagstaff said.
"Sadly, our construction sector has a terrible record of workers being killed or injured on the job."
Last year 11 construction workers were killed and 4800 workers were injured.
"The construction industry is a substantial and significant employer, and it's very important that people who work in this sector are kept safe," Wagstaff said.
The Wynyard Quarter site is being developed by Precinct Properties and Auckland Council's development arm Eke Panuku, where three new office buildings with ground floor retail are planned.
Construction started this year and is due to be completed in 2025.