The rangers were examining the success of predator control in protecting pekapeka from threats like stoats, rats and possums, as part of DOC's National Predator Control Programme.
DOC Biodiversity Ranger Sarah Wills says the size of the roost – 275 individual pekapeka – was staggering.
"Our rangers were buzzing at the discovery of such a massive roost. Being there and seeing the roost was a real privilege."
A typical pekapeka roost holds up to 100 individuals. The record of 358 bats was from a roost in a cave in the Te Kuiti-Waitomo area, monitored in the 1990s. The latest find is the largest recorded for a tree roost.
"We couldn't ask for a better demonstration of predator control benefitting our critically endangered pekapeka," says Sarah.
Pekapeka are only found in Aotearoa New Zealand. They most often shelter in trees during the day, usually choosing the oldest trees with large cavities to roost in. Colonies regularly move between different roost trees, so predator control needs to take place over large areas to protect them.
An intensive predator control programme has operated across Whirinaki Te Pua-ā-Tāne Conservation Park for more than 20 years, with DOC and Te Rūnanga o Ngāti Whare working in partnership to protect the species and ecosystems of Whirinaki.
"Predator control has made all the difference for species normally vulnerable to predation or browsing by stoats, rats and possums," says Sarah.
"We already know predator control is benefiting native species like whio, kiwi, and tōtara in Whirinaki, and it's fantastic to see it paying off for pekapeka across the wider area."
The programme includes aerially applied 1080 bait, as well as ground control methods.
The next aerial predator control operation, scheduled for winter this year, is expected to give native birds and pekapeka a greater chance to breed successfully in spring and summer.
