Helicopters employing night vision goggles have been working through the night to battle a major scrub fire near the Tiwai Point aluminium smelter.
The helicopters plus 11 fire crews are still trying to contain the blaze, which now covers around 130 ha.
Fire and Emergency shift manager Brent Dunn said this morning the fire was still a threat to the smelter's electricity lines.
Mr Dunn said crews would be able to get a better idea of how to reduce the fire once daylight breaks.
There will be a changeover of crews at 6am and at first light four more aircraft will join the fight.
Fire and Emergency said spokesperson Sally Chesterfield said the side of the fire closest to Tiwai was "contained".
The aluminium smelter is the country's single biggest consumer of electricity, using about 14 percent of the national supply, and fire damage to the lines could mean major disruption to its operations.
Fire a serious risk to smelter
Fire and Emergency said yesterday it received a call just after 7pm reporting smoke in the area. The said the fire was initially burning across about 6ha, but had spread to cover 100ha.
Just before midnight, a spokesperson said the blaze remained about the same size and still not contained.
Spokesperson Sally Chesterfield said there was no indication from crews the fire was getting any closer to the smelter, but it was being treated as a serious risk.
Eleven crews and four helicopters were earlier helping to fight the fire.
Witnesses in Bluff across the water said the fire was very large, and they could see huge plumes of smoke rising.
They said helicopters had been working non-stop to put the fire out.
Acting shift manager John Ditmer earlier said there was no indication any buildings were at risk from the fire, and no injuries have been reported.
A total fire ban was imposed on the whole of Southland eight days ago.