A teenager - possibly an apprentice - is believed to have accidentally left unattended the blowtorch that sparked the devastating SkyCity fire, bringing central Auckland to a standstill.

A well-placed source told the Herald the teenager, who was working for a sub-contractor, was called for a smoko break and only remembered when he was away from the roof that he may have forgotten to turn off the gas blowtorch being used to help install waterproof membrane.

The source understood he was returning to the roof to check the blowtorch but by then the fire had started.

"The poor guy is in tatters ... he's completely shattered."

It is understood part of the investigation will look at supervision and experience of the workers on the roof.

SkyCity Entertainment Group's chief executive Graeme Stephens said there were surveillance cameras operating in the precinct but he didn't know how the fire started.

He was however concerned about the worker who is alleged to have been involved.

"We have been worried about that person. We don't know who it is. There's certainly no witch hunt from our side. We have a genuine concern for their mental health and wellbeing.

"We're not trying to find them and get them back. We don't know the cause of the fire. We have concern for anyone involved. We're absolutely of that view. We need to get to the cause, obviously."

Asked if any video footage was available that identified how the fire started, he said: "We have got our normal surveillance cameras in place."

It is unclear where the teenager is now or if he has been officially spoken to about the events of last Tuesday.

A worker on the site told the Herald there were two young workers on the roof that day.

Despite being young and often working on their own they seemed to have a fair bit of experience, he said.

"From what we and what I saw...they were skilled guys. I wouldn't say that because they look young that they were necessarily apprentices. They might have been some of the more skilled guys on the team.

"I know when they lay the roofing, they actually - the way they worked and the way that they did the roof - they looked like they were very skilled and that they'd [been] doing it for quite some time."

The worker said there were up to 15 CCTV cameras around the building but the ones on level five and six were not pointed at the roof entry.

Read the full article on NZ Herald here