Maungatautari was chosen for this project because of what it still had, despite decades of damage. As an eroded volcano rising from the Waikato dairy country, its terrain offered diversity: lowland forest, montane bush, wetlands, and rocky outcrops. The surrounding farmland made fence construction feasible, and the local community threw its support behind the idea. In November 2003, the Maungatautari Trust built two trial exclosures totalling 1.1 square kilometres to prove the concept. By July 2004, construction of the Xcluder fence began in earnest. The design had to solve problems that no ordinary fence could: streams that fluctuated with rainfall, debris carried by floodwaters, and the need to allow native fish to migrate while keeping every mammalian predator out. Electronic surveillance systems monitored each watergate. By September 2006, the full 34-square-kilometre perimeter was sealed.

With the fence complete, the Trust began a systematic extermination campaign against every introduced mammal inside it. Brodifacoum poison was dropped in November and December 2006. Trapping and hunting supplemented the poison drops. The combined assault eliminated brown rats, black rats, stoats, cats, weasels, ferrets, red deer, fallow deer, pigs, goats, possums, hedgehogs, rabbits, and hares. Mice proved more tenacious. A third poison drop in September 2007 targeted them specifically, but it failed. By 2011, the Trust accepted that complete eradication of mice was not achievable and shifted to a control strategy instead. It was a rare admission of partial defeat in a project otherwise defined by its ambition, but the ecological gains were already extraordinary. Native beetle populations in the southern enclosure surged 300 percent within a year of predator removal.

Species began returning, some on their own and others with help. The Trust reintroduced critically endangered takahe in June 2006, endangered whitebait species in April 2007, and kaka that same year. Two kiwi eggs were discovered in September 2007, and one hatched that December, the first kiwi known to have hatched on Maungatautari in a century. By 2021, 320 North Island brown kiwi had been introduced. Tuatara arrived from Stephens Island in 2012. Mahoenui giant weta, North Island saddleback, kokako, and riflemen followed in subsequent years. Perhaps the most dramatic chapter came in July 2023, when four kakapo were released into the sanctuary, becoming the first kakapo on mainland New Zealand in nearly 40 years. The birds promptly tested the fence's limits: one escaped by climbing a downed tree and parachuting over the barrier with its wings. It was tracked by GPS and returned, but several more escape attempts led DOC to reduce the population to three.

Some of Maungatautari's most significant discoveries were not planned reintroductions but revelations of what had been hiding all along. In December 2004, eleven endangered Hochstetter's frogs were found living in a rocky area of the mountain, a population no one knew existed. In March 2010, a dead Duvaucel's gecko turned up in a mouse trap, marking the first confirmed sighting of the species on mainland New Zealand in nearly 100 years. The gecko's presence suggested a surviving population that had persisted in secrecy. And in April 2006, botanists found 100 silver beech trees on the mountain, a species native to southern New Zealand that had never been recorded this far north. The largest specimens were estimated to be several centuries old, relics of the last ice age. Each discovery underscored the same lesson: even in a landscape that seemed thoroughly catalogued, Maungatautari held secrets.

The fence is not infallible. Storm damage in July 2007 required quick repairs. Kakapo proved that even flightless birds can outsmart a barrier if the right tree falls in the right place. Mice still live inside the enclosure. But Maungatautari has demonstrated something that matters enormously for conservation in New Zealand and beyond: that a mainland ecosystem, surrounded by farmland and human activity, can be restored to something approaching its pre-human state if you are willing to build the infrastructure and do the relentless, unglamorous work of keeping predators out. From the air, the mountain is a conspicuous patch of dark green rising from the patchwork of Waikato pastures, its forest canopy unbroken and dense. What you cannot see from altitude is the fence threading through the bush at its base, or the birds calling from the canopy above it. You have to listen for them.