A century-old match factory was the likely source of a phosphorus dump which caused the evacuation of residents after it was discovered at a Caversham development on Wednesday.
Emergency services cordoned off part of David St and Rutherford St on Wednesday evening after the discovery of the volatile compound.
Hobby historian and David St resident Amanda Nunn said the Fire and Emergency New Zealand callout was to the site of an old match factory, which was likely the source of the phosphorus.
The Wax Vesta matchworks was founded in 1895 and shifted to Caversham in 1901.
It made strike-anywhere matches, which were banned in 1911 due to the danger they posed to workers and consumers.
A site neighbour said construction crews exposed an old sump when a digger scraped back the concrete covering it.
The crews did not seem to know the phosphorous was there and when the substance was exposed to air it ignited.
"Mayhem" ensued, the neighbour said.
The air smelled bad and the area was still slightly acrid yesterday, he said.
He was upset at how long it took for nearby residents to be notified about the situation.
Another resident said it was not the first time emergency services had been called to reports of smoke at the development.
In an incident last month he had noticed it was "pretty foggy outside".
Then he turned around saw a fire.
He believed the blaze was caused either by heat from an excavator sitting on a pile of debris, or embers left over from cutting steel beams earlier in the day.
The fire was sorted by emergency crews pretty quickly, he said.
University of Otago chemistry professor Lyall Hanton thought the substance uncovered was probably white phosphorous.
White phosphorous will slowly oxidise over time and with contact with water, becoming quite benign.
But in the case of a large cache it would not all be oxidised by decades in the ground.
The developers declined to comment yesterday.
