The release of a captive population of the rare Hamilton’s frog into the wild at Wellington’s Zealandia ecosanctuary has marked a significant milestone in the survival of one of the most endangered frogs in the world. Words Annie Studholme.

Boasting the sole mainland population of Hamilton’s frogs living in the wild, Zealandia Te Māra a Tāne ecosanctuary, located just minutes from Wellington’s city centre, provides visitors with a unique opportunity to catch a glimpse of New Zealand’s rarest and largest frog outside of our remote, offshore islands.

Like many of New Zealand’s indigenous species, frogs (pepeketua) have lacked the crowd appeal of cute mammals and birds, so their story remains largely unknown. Some of the most unique and diverse species on the planet, frogs survive in a wide variety of environments, but it’s the role they play in the grander scheme that makes them critically important. Regarded as an ecosystem’s danger sign, a frog’s permeable skin is very sensitive to environmental changes, pollutants, poisons and disease, making them excellent indicators of the overall health of their ecosystem. They are also critical to the food chain, acting as predator and prey.

Worldwide, amphibians are facing a global extinction crisis, with more than a third of species threatened with extinction. Of the nine endemic species once present in New Zealand, six are extinct, and the remaining three – the Archey’s, Hochstetter’s, and Hamilton’s – are all threatened. ‘People really have no idea what a crazy, weird and wonderful species they are,’ says Zealandia General Manager of Conservation and Restoration, Jo Ledington.

Together with the tuatara and kiwi, New Zealand’s frog species are truly unique. They belong to the genus Leiopelma, an ancient and primitive group of frogs that floated away on prehistoric New Zealand 70 million years ago and has changed little since. Not even any of Australia’s several frog species are similar.

However, when people think of frogs, they’re probably talking about our Australian imports, who were introduced in the middle of the last century, including the green and golden bell frog, the southern bell frog and the whistling tree frog. ‘If you hear a frog singing in your garden, it’s likely to be an introduced species from Australia, not a New Zealand native,’ says Jo. ‘Our native frogs don’t have vocal sacs and therefore don’t croak regularly (although they can still make tiny chirps when alarmed).’

But that’s just the beginning. These tiny nocturnal, ground-dwelling species also have big round (not slit) eyes, no external eardrums, are camouflaged to near invisibility and live amongst rock piles and leaf litter, so they have almost no webbing on their feet. They also don’t have a tadpole stage, preferring to lay their eggs on the ground. The males guard these eggs for several months until the tiny (about the size of your little fingernail) froglets hatch. They then carry them around on their backs until they are fully developed. Research suggests it’s not unusual for Hamilton’s frogs to reach more than 40 years old, spending their entire life living in about one square metre, Jo explains.

A frog’s permeable skin is very sensitive to environmental changes, pollutants, poisons and disease, making them excellent indicators of the overall health of their ecosystem.

Though fossil records indicate that Hamilton’s frogs were once widespread throughout the lower North Island and upper South Island, at one point, only two small populations in the Marlborough Sounds survived due to habitat loss and invasive predators. Sixteen hectares of steep bush on Maud Island was fenced off from stock to protect the frogs in 1965, with the Crown purchasing the entire island in the late 1970s. Regular monitoring showed the population was flourishing, but there were concerns fire could be catastrophic, wiping out the entire population.

The first Hamilton’s frogs from Maud Island and a captive population at Canterbury University were translocated to Zealandia in 2006 to create an ‘insurance population’. Zealandia is the world’s first fully fenced ecosanctuary covering 225 hectares of inner Karori. Since the completion of the fence in 1999, the sanctuary has reintroduced native wildlife back into the region, some of which had been missing for over a century.

To begin with, the tiny frogs were released into fully enclosed pens as it wasn’t known how they would fare with the kiwi and low levels of mice present in the sanctuary. Initially, the 30 males and 30 females were kept separate. The first breeding occurred in 2008. In 2012, a further 100 frogs were released outside one of the pens within a kiwi-proof fence to see how they would do in the presence of mice while still protecting them from kiwi.

Despite being notoriously tricky to monitor, surveys conducted at Zealandia over the past few years have found frogs of varying sizes. ‘Frogs slowly get bigger as they age. The females grow bigger than the males. The biggest animal we measured was seven grams. We have also found pregnant females and young froglets in captive and wild populations, indicating the population is doing well,’ says Jo.

Following conversations with the Department of Conservation (DOC) Frog Recovery Group and with the support of kaitiaki Ngāti Kuia and Taranaki Whānui ki Te Upoko o Te Ika, it was eventually decided the frogs could be released. To ensure the frogs’ safety, a new boardwalk was built using funds generously donated by the estate of John Nankervis to protect them from human foot traffic.

The release of the last captive population in August 2024 was the direct result of nearly two decades of studies, surveys and research involving dozens of people. Its success is a huge boost to conservation efforts across the nation. ‘It’s exciting to see the hard work put in by so many come to fruition. Every piece of this research has fed into this decision – years and years of perseverance. Every piece of information has helped us get to this point. Now, with the species fully wild, we can also explore complex questions that will have further implications for the conservation of this species at a national level,’ Jo concludes.

The newly released population of Hamilton’s frogs will continue to be monitored by Zealandia as they disperse.