A major renewal of the Makatote Viaduct on the North Island Main Trunk railway line is one of KiwiRail's biggest projects at the moment, but precious wildlife in the immediate vicinity isn't being overlooked.
The viaduct spans the Makatote River on the western side of Mt Ruapehu and south of National Park, but the riverside beneath the 80m tall structure is also home to the rare whio (blue duck) and other native fauna.
So while the steelwork is being sandblasted, repainted and in some cases renewed, KiwiRail has given the Department of Conservation money to protect the wildlife for the next five years.
Mike Keenan, KiwiRail project representative, said the $13 million viaduct project will be finished at the end of next year but the corporation wants the protection to continue beyond that point.
"We're committed to protecting the surrounding environment and birdlife and we recognise the ecological importance of this area," he said.
Mr Keenan said he did not have an exact figure but he understood the protection programme could cost between $86,000 and $110,000.
Steve Moorhouse, a Department of Conservation ranger based in Ohakune, said DoC was not trapping pests at the viaduct.
"But the money and support we've had from KiwiRail has been great for us. They're committed to not just the trapping but also protecting the river from any potential fallout from the sandblasting and painting that's happening on the viaduct," he said.
Mr Moorhouse said they had run traplines along both sides of the Makatote River, each trap about 100m apart, to catch rats, stoats and ferrets.
He said this style of trapping had been particularly good in areas where whio were living and was an effective way of controlling pests where the ducks were nesting.
He said there was only one breeding pair in the area around the railway viaduct.
"They're a very territorial bird and generally have a range of 1-3km from their nesting area."
He said the pair near the viaduct had ducklings so were breeding quite successfully.
This is not the first time the birds have been subjected to special protection measures.
When footings on the viaduct were reinforced in 2006, an elaborate set of silt traps was installed around the construction site to protect the habitat.
The river also has a healthy number of brown and rainbow trout.