A firefighter says a large blaze on conservation land in Southland has been contained.

The fire in the area around Awarua Bay, about 20km south of Invercargill, was reported about 6.15pm on Monday and stretched over 400 hectares at its peak.

Initial reports suggested the fire could take days to put out but Southland deputy principal rural fire officer Elton Smith says the team of three helicopters and multiple rural fire crews had been successful by early Tuesday afternoon.

"It's all contained and most of the helicopters have been stood down. We're just mopping up," he told NZ Newswire.

"We'll have another crew out here tomorrow just tidying up hot spots and that sort of thing but that's pretty much all that's left now."

The firefighters had concentrated on containing the fire on the northern side to stop it reaching Waituna Lagoon before taking on the southern side on Tuesday morning.

"It's gone bang on plan," Mr Smith said.

All the helicopters have been stood down, bar one Mr Smith says is "ferrying firefighters around the perimeter".

The blaze had not threatened any buildings but the area has high conservation value.

The Department of Conservation website describes the Awarua Wetlands as one of the largest remaining wetlands and is recognised for its biological diversity and cultural values.

The wetlands include coastal lagoons, freshwater swamps, extensive peatlands and estuaries.