Experts discovered arsenic in the smoke plume that drifted over Auckland CBD during the SkyCity international convention centre fire.
GNS Science tested samples taken by Auckland Council and a university researcher close to the fire site.
The tests measured levels of PM, or particulate matter (PM10) the term for a mixture of solid particles and liquid droplets found in the air.
It was found that for the first time in a decade, the air quality in central Auckland exceeded national standards on Wednesday 23 October, the second day of the fire.
Experts said there were elevated levels of black carbon, zinc and arsenic within the smoke plume.
The council said in a statement that this only occurred inside the smoke plume and when public alerts were in place advising people to avoid exposure to the smoke.
"These results were expected from a large building fire, as construction materials like bitumen, treated and untreated timber all release particulate matter when burned," the statement said.
"The levels of arsenic, resulting from the burning of treated timber, are not likely to result in significant health impacts given the relatively short time the levels were elevated."
It said the public were alerted and the air quality was back to normal the next day.
"We recommend people always take appropriate precautions where there is smoke from any fire including making sure homes are well ventilated, ensure chimneys and fireplaces are regularly checked for safety, and never burn treated timber," the statement said.