The successful restoration of the viaduct follows decades of neglect.
More than 50 guests, including Conservation Minister Eugenie Sage, have been flown by helicopter into the remote world heritage area to celebrate the reopening of Percy Burn Viaduct - a bridge that spans the valley.
Percy Burn is regarded as one of the world's largest wooden mill tramway viaducts and is recognised as a Category One Historic Place by Heritage NZ Pouhere Taonga.
It now provides a vital link in the Hump Ridge and South Coast tracks, which could prove a boon for tourism in the region.
Ms Sage used the South Coast Track when she tramped in the area in the late 1990s. The calls of kākā while walking along Percy Burn Viaduct over world heritage forest made it a special memory, she said.
Musicians played as guests wandered along the viaduct this morning before being welcomed with a karakia, speeches, and birdsong.
The ceremonial ribbon was removed a bit earlier than expected when several trampers passed the bridge.
About 4000 people visit or pass over the structure each year, but that's slated to increase dramatically if the Hump Ridge track is approved for 'Great Walk' status by the Department of Conservation. It's currently in the top three.
The restoration work has been led and fundraised by Southland's Port Craig Viaducts Charitable Trust, and has been completed for about $750,000.
A three-way partnership between the trust, Department of Conservation and Southland District Council was set up in 2015 to undertake the work.
Trust chairperson Tom Pryde said it was a challenging project to undertake.
"The viaducts were originally designed to last 30 years and no-one would have imagined that they would still be standing, let alone be enjoyed by walkers on the Hump Ridge Track almost 100 years later," Mr Pryde said.
Percy Burn Viaduct forms part of a heritage precinct of four other viaducts. They date back to the 1920s as part of an indigenous logging and milling operation run out of Port Craig, a frontier town without road access.
By the early 1990s, Percy Burn was in danger of collapse after decades of neglect and an arson attack, which had burnt through several trestles.
The 36 metre-high bridge was closed in May 2013, amid fears of structural safety. That meant the tracks using the iconic viaduct were rerouted.
"This is a significant project when we consider the age of the structures and the magnitude of the rebuilding necessary as well as the remoteness of the worksites in Southern Fiordland," Mr Pryde said.
"It hadn't had any restoration work done on it of any kind for 70 years. It was really, really dangerous. If we hadn't restored it that year when we did in 1993, it absolutely would have been demolished. They would have blown it up because it would have been too unsafe it was still in the same state."
Ngarita Dixon was part of Southland's Port Craig Viaducts Charitable Trust, which led the charge to restore the viaducts.
"It astounds me because you've got to remember this was built in the 1920s and that was the wheelbarrow and the shovel era," Ms Dixon said.
"They built this thing that's longer than a football field and higher than a six-storey building with nothing like we've got today to use to build it."
Ōraka Aparima Runanga general manager Riki Dallas said reopening the viaduct allowed New Zealanders to explore what was on their backdoor step. His ancestors walked along the paths near where the viaduct stands, he said.
Venture Southland's Steve Canny said the restored viaduct would hopefully boost their chances of becoming a Great Walk.
"The access along on the south coast via the viaducts opens up all these possibilities," Mr Canny said. The restoration was a remarkable achievement for a passionate community, he said.
The final decision on Hump Ridge Track's Great Walk status is expected to be announced in the next couple of months with hopes the track will be ready for the 2021 season.
The work was completed by Fulton Hogan with engineering support from Queenstown's Derek Chinn.