Firefighters are worried a wind change today could see a large blaze on conservation wetlands at Awarua, south of Invercargill, spread further.
The fire, at Awarua, south of Invercargill, has burned through a thousand hectares of mānuka scrub and peat soils.
Nine helicopters and eight ground crews were expected to resume fighting the fire at 8am, with heavy machinery used to create fire breaks.
Incident controller Mark Mawhinney told Morning Report there was improvement on yesterday morning.
But "the potential for this to destroy a much bigger piece of this wetland is certainly there" as a change in wind direction is forecast for this afternoon.
This will bring easterly gusts up to 45 knots, blowing the fire toward unburnt land.
"That really could change everything, all the hard work we've done could be lost. So a real focus this morning to put as much effort into this fire as we can," Mawhinney said.
"That wind change has got us worried, but we're doing everything we can to get these edges out and hopefully once that wind change comes through we'll also be ready to do what we need to, to stop it from spreading."
It was likely to keep burning for some time to come, he said.
"That's our big concern, Southland is in a drought, so on top of that, a fire like this down into those deep peat layers, we are planning for a couple of weeks here to manage this fire.
"We're a long way from being comfortable that this fire is safe."
MetService forecaster Tui McInnes said a front bringing patchy rain was expected to move in later this afternoon.
"The first couple of fronts, they do pass over the deep south later today. With that will be cloudier conditions and possibly a bit of rain. It's unlikely to be anything that's going to be persistent ... we're looking at sort of brief and light."
McInnes said it would mainly be overcast, with significant rainfall not expected until tomorrow evening.
The rain would become more persistent over tomorrow and Wednesday as the front turned southerly, he said.
Investigators were trying to determine the cause of the fire, but had no clues so far.
The area where the fire is burning is remote, with just a few huts and holiday homes nearby, but none are threatened so far.
Awarua-Waituna is one of New Zealand's largest remaining coastal wetland systems, one of five significant wetlands in the Department of Conservation's national Awarai Kākāriki wetlands restoration programme. It is home to many wading birds and other species.