A fire in a small bin at the Commercial Bay complex above the newly opened shopping centre was sparked by the spontaneous combustion of used cleaning rags.

Fourteen fire trucks rushed to the Auckland CBD late Tuesday night after a fire broke out on the 22nd floor of the multimillion-dollar building.

Fire and Emergency said a fire in a small skip on the high-rise construction site triggered the building's sprinklers and alerted emergency services shortly after 11.15pm.

Firefighters found smoke spreading through the lobby, activating a third alarm.

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A Fletcher Construction site manager was being spoken to by Fire and Emergency, who told the Herald they had no contractors on site when the fire began.

But yesterday a Fire and Emergency spokeswoman said the fire appeared to have sparked by itself.

"It appears the fire, which was contained to the skip rubbish bin, was caused by spontaneous combustion of some oily cleaning rags which were in the bin," she said.

The fire appeared to be caused by the spontaneous combustion of oily rags inside the rubbish skip. Photo / Will Trafford
The fire appeared to be caused by the spontaneous combustion of oily rags inside the rubbish skip. Photo / Will Trafford

At the scene late on Tuesday night, assistant area commander Roger Callister said it appeared a sprinkler system installed on the floor had helped to stop the fire from spreading any further.

"The sprinkler system had activated and extinguished most of that fire," Callister said.

"Our crews started looking for hotspots within the skip bin and tried to start removing the smoke."

The fire did not affect the retail precinct, which was still open and trading.

Fourteen crews attended the fire - from Auckland City, Parnell, Grey Lynn, Balmoral, Remuera, Mt Roskill, Ellerslie, Te Atatū, Birkenhead and Takapuna fire stations.

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Operation support, executive officers and an investigator also attended.

Fourteen crews from across Auckland attended the blaze. Photo / Will Trafford
Fourteen crews from across Auckland attended the blaze. Photo / Will Trafford

Shortly before midnight, Fire and Emergency services northern shift manager Craig Dally said, the blaze was out. A spokesman said this morning that fire crews left the scene about 1.30am.

There were no reports of injuries.

The shopping centre on the bottom floor of the building opened last month and was hailed as a chance for the struggling retail sector to rebuild after the Covid-19 pandemic.

Trading at Commercial Bay got off to a booming start, with almost a quarter of a million people visiting the downtown shopping centre in its first four days.

Prime Minister Jacinda Ardern officially opened the 18,000sq m retail and dining precinct in the Auckland city centre on Thursday June 11.

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What is spontaneous combustion?

Like any fire, three elements are needed for spontaneous combustion: heat, fuel and oxygen.

Some oils - such as vegetable, drying oils and biodiesel - undergo the chemical process of oxidisation when exposed to air, releasing energy in the form of heat.

This oxidation creates the potential for spontaneous combustion - to start a fire without warning.

If you have oily rags covered in varnish, for example, as the varnish dries, this chemical reaction creates heat.

If these rags are tightly scrunched up in a bin, this chemical reaction can produce enough heat to spark alight.

The oil and absorbents become the fuel while oxygen is present in the air.