A union boss is questioning health and safety procedures in the construction industry after an explosion at a building site in Auckland's Wynyard Quarter.

First Union General Secretary Dennis Maga said, "Employers working in the construction industry need to have a robust health and safety procedure. This is basic stuff."

"Employers will complain because of technical concerns and costs for them, but we need this in the workplace to make sure people are safe," he said.

The union represents workers in the construction industry, however, no one working at the Hawkins construction site was a member.

Maga said construction employers should be audited on their health and safety requirements for all those operators working on their sites.

"Big employers should practice due diligence or else they're simply passing on responsibility to subcontractors. The audit procedure must be robust because we're dealing with people's lives here," he said.

Dennis Maga, First Union general secretary, said construction industry employers need to have "robust" health and safety procedures in place after an explosion in Central Auckland. Photo / NZME.

"If they have LPG gas on site, does the employer have no-smoking policies in place or have designated smoking areas? And then, who's checking that equipment is turned off [at the end of a workday] or in the morning?"

He said employers needed to ensure workers properly understood the health and safety procedures in place. Proper health and safety procedures needed to be tailored to workers, he said, some of whom were migrants and knew little English.

Maga said feedback First Union received from their members was that sometimes they're afraid or unsure how to ask their foreman questions during toolbox meetings.

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It is suspected that a gas cooker caused Friday morning's explosion, injuring five people. Two remain in critical yet stable conditions, according to a Counties Manukau Health spokesperson.

Maga said, "we support a full investigation like what [the Council of Trade Unions] has called for."

CTU president Richard Wagstaff 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."

Wagstaff called for an investigation into the explosion, saying "it's important there be a full and detailed investigation into how this happened and steps are taken to better protect workers".

A spokesperson for Worksafe said any information would be "months away" and couldn't give any further updates on the explosion.

Helene Toury, spokeswoman for Hawkins said no further information could be given until investigations were completed.