Hydrological state and trends for Tāmaki Makaurau / Auckland 2025. State of the environment reporting.

Executive summary

For the State of the Environment 2025 (SOE2025) hydrology report, rainfall, soil moisture, river flow and groundwater level observations were analysed for the most recent five-year period ending in mid-2024 and further back in time to evaluate long-term trends.

Key findings for climate conditions:

• A significant and protracted La Niña episode unfolded during SOE2025 and it influenced Auckland’s regional climate and weather conditions. Hydroclimatic observations over the SOE2025 period (mid- 2019 to mid- 2024) shows the Auckland region experienced some of the most significant hydroclimatic variability and extreme events since instrumental observations began in the mid-1800s. Important short-term rainfall variability shown during SOE2025, with seasons that experienced 100s of millimetres more or less rainfall than normal, is strongly apparent for Auckland, and that variability exists on top of emerging long-term rainfall trends.
• There are subtle long-term rainfall trends, but many were not statistically significant using standard linear trend approaches employed for SOE2025 reporting. Rainfall observations at Whenuapai, where there is a long record, showed a reduction in autumn rainfall and lower overall hydrological year rainfall amounts since the mid-20th century. This drying trend is in line with regional atmospheric circulation changes, and climate change projections for Auckland that indicate an increasing frequency of dry days emerging during the 21st century.
• Short-term climate variability observed for SOE2025 resulted in lower overall rainfall in autumn for most years relative to the years in SOE2020 (2014-2019). Multiple dry summer seasons were observed in the north, west and central part of the region during SOE2025. The relatively dry autumns observed for SOE2025 are notable because if they become more frequent, then more significant overall drying will occur if climate change projections of drier spring conditions in the future also arise.

Key findings for rainfall observations:

• Exceptionally high rain amounts across multiple seasons led to an extraordinarily wet 2022-23 hydrological year that saw multiple sub-daily, daily, multiday, monthly, seasonal and annual rainfall records broken. The Auckland Anniversary 2023 storm event was a significant factor for setting new rainfall records. The influence of individual seasons (like autumn 2023) was important for determining overall short-term climate averages at some sites. Isolated significant long-term trends for increased winter and spring rainfall were identified along the southern margin of the Auckland region.
• Succession of rain events in some seasons and the absence of rainfall that led to drought in others also highlights the importance of observing rainfall continually over the long-term. Overall, the local and regional meteoric water balance for SOE2025 at many locations may have been much lower without several distinct, very strong and exceptional rainfall events that occurred in the past five years.
• The 2019-20 drought was the most severe that has occurred since instrumental observations began in the mid-1800s. There has been an increase in the number of dry spells for Auckland and evidence that dry conditions and drought are becoming more common in spring alongside rising temperatures for the region. These observations are consistent with new climate change projections that indicate an increase in the number of dry days during spring with reduced rainfall in that season, as well as an increased frequency, intensity and duration of drought for northern regions of New Zealand.

Key findings for soil moisture and river flow observations:

• During SOE2025 there was high variability of soil moisture and pronounced shifts in both high and low river flow metrics, with 80% of monitoring sites observing new peak instantaneous flows. The 1% flood Annual Exceedance Probability (AEP) was exceeded at all sites during SOE2025. The Auckland Anniversary 2023 storm caused over half of the observing sites to show more than a 10% increase in 1% Annual Exceedance Probability (AEP) for flood frequency values.
• 49% of observing sites experienced their lowest or equal lowest flow during SOE2025 and of those about 3 out of every 4 observing sites experienced their lowest flow or lowest equal flow during the 2019-20 hydrological year when the significant drought occurred.
• An increase in FRE3 (frequency of flow events greater than 3 times the median flow) was observed at six sites (mostly in urban catchments) while decreases in FRE3 were observed for six catchments in mostly rural or native forest settings. Significant linear trends were observed for the mean annual low flow at multiple sites, but some of the observed changes were very small. Otara showed a significant increase in the annual low flow level across all time periods both with and without 2022-23 data included.

Key findings for groundwater observations:

• Significant negative trends for groundwater levels at Maraeorahia and at Quintals suggest there could be connections to long-term climate changes as well as groundwater abstraction. A decreasing trend at Maraeorahia shows a trend toward fewer months of groundwater surplus through time during the main recharge season. Declining groundwater levels along with groundwater level variability that aligns with rainfall and soil moisture deficits highlights the vulnerability of groundwater resources to drought.

There will be repeated climate and hydrological impacts that continue to place pressures on our water-sensitive assets like stormwater systems, parks and public spaces and our water supplies. Maintaining and improving the current hydrology observation network to enhance understanding of both short- and long-term changes for the Auckland region will support the development of early warnings about extremes (like storms that can cause flooding) and climate events (like drought) that can help to minimise impacts and inform on water resource management strategies for agriculture and reticulated water supply schemes.

Recommendations for the development of future observations that can support and enhance SOE reporting:

• a more detailed view of soil type spatial variation, soil moisture changes at depth, and more coverage across the region including approaches that combine observations, modelling and remote sensing,
• additional groundwater aquifer monitoring bores that are distributed across recharge areas, central locations in the aquifers and near discharge areas or located down-gradient near aquifer boundaries,
• high-quality groundwater use and groundwater consents data, including water level and volume usage,
• additional statistical and trend detection approaches, and new composite and synthetic data series for robust analysis of hydrological and hydroclimatic change,
• ‘Sentinel hydrological monitoring sites’ where a full suite of observations (including lysimeters andadditional soil moisture sensors) to inform on hydrological processes more completely.

Auckland Council technical report, TR2025/27

September 2025

See also

Te oranga o te taiao o Tāmaki Makaurau. The health of Tāmaki Makaurau / Auckland’s natural environment in 2025. A synthesis of Auckland Council’s state of the environment reporting