Three historic wooden viaducts in southern Fiordland remain closed this long weekend after safety inspections revealed rapid deterioration of the century-old structures.

Percy Burn and Edwin Burn viaducts on the Hump Ridge Track, and Francis Burn Viaduct on the South Coast track, were shut down in February due to structural concerns.

Despite the closures, the three-day Hump Ridge Track remains open to walkers. Hikers can use existing alternative tracks and bridges to bypass the closed Percy Burn and Edwin Burn viaducts, while the fourth structure, Sandhill Viaduct, stays open.

However, there's no marked alternative route around Francis Burn Viaduct on the expert-level South Coast track, as the surrounding gorge is too steep for safe passage.

DOC Te Anau Operations Manager John Lucas says the structures have withstood harsh southern conditions for more than a century, but time has caught up with them.

"These impressive wooden structures have borne the brunt of the wild southern conditions for more than 100 years. Timber always has a finite lifespan, and recent inspections showed the hardwood structural components have deteriorated rapidly in recent times. The viaducts are not safe to cross," Lucas said.

The viaducts form part of the Port Craig Viaduct precinct, considered one of New Zealand's most significant heritage sites. Lucas says heritage conservation advice will guide future decisions about the structures.

"Safety must come first, so the decision to close them while we work with the Southland District Council, who own three of the four viaducts, and the Hump Ridge Track on future management options was an easy one," he said.

DOC conducts six-monthly safety assessments on all structures. When the Hump Ridge Track was upgraded to Great Walk status, a 58-metre swing bridge was built as a backup for the Edwin Burn Viaduct.

Visitors planning weekend trips should check the DOC website or local visitor centres for current track conditions.

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