Executive summary

Kauri dieback disease and the pathogen Phytophthora agathidicida (P. agathidicida) have been detected in most regions where kauri grow in Aotearoa New Zealand. Once established in a forest system, the pathogen cannot be eradicated and infection often results in the death of kauri trees.

Auckland Council has made significant investments into both kauri protection and P. agathidicida delimiting surveillance since 2009. In 2021, the Waitākere Ranges Kauri Population Health Monitoring Survey determined pathogen and disease prevalence across the kauri population and set a baseline for future assessment of change. The 2023 Te Ngāherehere o Kohukohunui / Hūnua Ranges Kauri Population Health Monitoring Survey was similarly designed to survey for symptoms of kauri dieback disease and monitor kauri health and had the additional aim of establishing whether P. agathidicida was present in the Hūnua Ranges.

The Hūnua survey was co-designed by Auckland Council, Department of Conservation, and ngā iwi mana whenua o Te Ngāherehere o Kohukohunui Ngāi Tai ki Tāmaki, Ngāti Tamaoho, Ngaati Whanaunga, and Ngāti Tamaterā.

We collected baseline kauri tree health, kauri dieback disease symptoms, potential risk factors, and ecological impact factors, and conducted soil testing for 561 kauri trees. We also conducted LAMP (Loop Mediated Isothermal Amplification)-based stream baiting in 20 stream locations within the study area. We found no evidence of P. agathidicida in the Hūnua Ranges and the extent of testing gives us 97-99.9 per cent confidence that we would have detected it in the study area if present at a prevalence of 1 per cent or more. This confidence is extremely important for informing ongoing forest management between all partners and landowners in Hūnua.

More than 95 per cent of the kauri surveyed were very healthy. More than 92 per cent of sites surveyed had seedlings or saplings beneath the monitored trees, indicating a healthy population with good recruitment. This was a much higher rate than the 55 per cent of sites observed in the 2021 Waitākere survey.

Kauri appear to be more prone to poor health in places that have been disturbed and these trees may be more vulnerable to disease in the event of P. agathidicida introduction. In Waitākere, the detection of P. agathidicida was strongly associated with historical and contemporary disturbance events, and in those places, kauri are in poor health and many are dying. The results of both studies suggest that minimising disturbance to the forest, especially to kauri root systems, is important for kauri health and general resilience.

We have successfully built a risk profile for Hūnua that identifies areas of highest risk for future introduction or detection of P. agathidicida, enabling partners to target introduction and subsequent spread prevention. This will also inform protected areas strategies around identified high-risk areas.

We now have a baseline of kauri health which can be used for ongoing monitoring, considering both the risk of introducing P. agathidicida and the detection of other potential impacts on kauri.

Monitoring current kauri health is essential to track any change over time and allow adaptive management. Long-term health monitoring will also help us determine how other factors affect kauri health, such as land use, environmental management, and climate change.

Ngā iwi mana whenua o Te Ngāherehere o Kohukohunui advocate for rāhui should P. agathidicida be detected and support ongoing monitoring, cleaning stations, pest control and exploring other initiatives to ensure the Hūnua Ranges remain free of P. agathidicida.

In conclusion, we did not detect P. agathidicida in the Hūnua Ranges, and the kauri population in areas of low disturbance are in good health. As most other large kauri forests have P. agathidicida infection, this study has highlighted the importance of the Hūnua Ranges for kauri protection. Long-term monitoring of kauri in Hūnua is critical for adaptive management and to prevent pathogen spread within-forest, should the establishment of P. agathidicida occur.

Auckland Council technical report, TR2025/1

January 2025.