A large wildfire in Southland is threatening electricity supply to the country's aluminium smelter.

The fire at Tiwai Peninsula near Bluff burned through the night after it began just after 7pm yesterday.

The fire has now claimed 100ha of native scrub.

Nearly a dozen fire crews have been battling the blaze as well as two aircraft using night vision googles because of the proximity of the smelter.

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Fire and Emergency southern shift communications manager Brent Dunn said the fire was still posing problems for the smelter.

"There is still a threat to the overhead transmission lines. The fire is still active but at this stage it is being well managed," he said.

Last night the fire was 100m from the main powerlines that run to the smelter, and 400m from the smelter itself.

Dunn said crews would be rotated in coming hours just before the blaze hit the 12-hour mark.

Dunn said the fire was difficult to reach as there were no roads into the area. Four helicopters with monsoon buckets were first to tackle the blaze.

"Its been a long fire. We've been trying to protect the Tiwai smelter, which they're doing a great job of and firefighter operations will just continue," he said.