Firefighters are battling a blaze at a hydro outfall at the old paper mill in Mataura and authorities are working to determine if there is any risk caused by the ouvea premix stored there.
The fire is at the hydro outfall, which used to power the paper mill, but is not the part of the site where the premix is stored.
A Fire and Emergency New Zealand spokeswoman said access to the site was difficult and through a small manhole, and firefighters were waiting on a caretaker to arrive to get access and determine the extent of the blaze.
In a press release police said the blaze was "large"
Emergency Southland manager Angus McKay said police and fire crews were still accessing the site to check if the fire presented any danger to the premix.
"The initial reports are that the fire started at the hydro-generator plant.
"The building is right next to the site [where the premix is stored] but it has some kind of separation between."
He could not inform how much ouvea premix was still stored in the site, but said it was a "considerable" amount as the work to remove the substance would take years.
In the end of May, Gore District Council chief executive Stephen Parry said about 8 per cent of the ouvea premix at Mataura had been removed so far.
Parry said the work was expected to be completed in two and a-half years but the first 12 months would be the most challenging.
"I don't have the exact figures but [I believe] it would be about between 750 and 800 tonnes that had been removed by now."
The work — which started in October last year — was expected to be completed in two and a-half years.
A Fenz spokeswoman said two appliances from Mataura were at the scene of the fire as of about 1.50pm, and two more were on the way from Gore.
The building was involved in fire, the spokeswoman said.
A reporter at the scene said it appeared to be at the paper mill. There were three fire appliances were at the scene and traffic was being directed from the area by police.
The reporter could not see any smoke, but firefighters were entering the building wearing breathing apparatus.
Police said road closures and a number of detours were in place due to a "large" fire on Kana St (SH93). Traffic was being diverted up SH1.
Motorists were asked to avoid the area and expect significant delays. The fire was reported about 1.40pm.
Toxic aluminium dross is stored at the site, which residents have long been concerned about. The concerns were heightened when the area was flooded in February.
The dross, a byproduct of aluminium manufacture at the Tiwai Point smelter, could produce poisonous ammonia gas if it came into contact with water.
It is stored in a former paper mill beside the Mataura River; community fears about the dross have been heightened by floods in the town in 2018 and in February this year.