SummaryNew Zealand's native frogs (genus Leiopelma) display a very high degree of endemism and belong to a distinctive and ancient evolutionary lineage. All four extant species are considered to be threatened or endangered, but the mechanisms behind their decline are poorly understood. Many of the potential factors causing population declines are confounded for the two species living on the mainland; in particular, habitat degradation and the presence of introduced mammalian pests have been difficult to disentangle in the past. Here we present data on a population of Hochstetter's Frog (Leiopelma hochstetteri) living in a pest‐free mainland sanctuary, the 3363‐ha Maungatautari Ecological Island. Survey data from this population shows a fourfold increase in numbers between 2009 and 2012 from a very low initial density. Our results suggest that for Hochstetter's Frog at least, and possibly the other mainland species of Leiopelma, introduced mammals are a major driving factor in population declines. Given that the frog population at Maungatautari was discovered by serendipity when planning for the pest‐free sanctuary was already well underway, this also highlights the conservation value of large pest‐free areas and ecosystem‐level management.
