Two workers remain in a critical condition today after suffering burns in the explosion at an Auckland waterfront construction site.

Five people were taken to Middlemore Hospital – home to the unit that cares for some of New Zealand's most severe burns patients – after the blast at a Hawkins wharf building site in Wynyard Quarter on Friday morning.

The cause of the explosion is thought to be a gas cooker which was left on overnight.

A Counties Manukau Health spokesperson told the Herald this morning two patients were in a critical but stable condition.

Three other people injured in the explosion were in a stable condition.

Some of those injured in the incident had their clothes and footwear blown off them by the power of the explosion.

The first firefighters on the scene doused those suffering from serious wounds with water from fire hoses.

In the aftermath of the explosion, a construction company staff member said they were a tight team and the firm's immediate focus was on the injured workers, their families and those they worked alongside.

Hawkins spokeswoman Helene Toury earlier confirmed the incident occurred at their site.

"Our immediate focus is to support the injured workers, their whānau and the team onsite. We are working with the relevant authorities to establish exactly what occurred."

The company was grateful for the efforts of the first responders - Fire and Emergency New Zealand, the New Zealand Police and St John New Zealand, Toury said.

Some of the first firefighters on the scene yesterday doused the burned workers with water from their fire hoses. Photo / Michael Craig

"We are a tight team and our thoughts are with the injured workers and their whānau."

The company couldn't comment on today's events until the incident investigation was completed.

A union leader called for a "full investigation" into the incident, saying "every New Zealander deserves to know they will be safe at work".

NZ Council of Trade Unions' president Richard Wagstaff said it was lucky no one was killed.

"Clearly on this occasion the health and safety system operating at this workplace has not kept workers safe and needs to be put right as quickly as possible," Wagstaff said.

"Sadly, our construction sector has a terrible record of workers being killed or injured on the job."

Eleven workers were killed in construction last year and 4800 had an injury requiring more than a week away from work, he said.

"Clearly on this occasion the health and safety system operating at this workplace has not kept workers safe and needs to be put right as quickly as possible.

"It's important there be a full and detailed investigation into how this happened and steps are taken to better protect workers."

Emergency first responders at the scene of Friday's construction site explosion which injured five workers. Photo / Michael Craig

The construction industry was a substantial and significant employer.

"WorkSafe must act swiftly to get to the bottom of what has gone wrong."

WorkSafe opened an investigation.

A spokesperson said as it was currently making inquiries as part of the investigation it couldn't comment further.

The incident triggered a large-scale response from emergency services who raced to the Halsey St site about 6.30am.

Work had not started at the Hawkins construction site on Pakenham St West when the explosion occurred. Workers were sent home and the site was closed yesterday.

- NZ Herald