Executive summary

Water temperature is a critical driver of stream ecosystem health, influencing dissolved oxygen levels, chemical processes, and the physiological responses of aquatic organisms. Elevated temperatures can cause thermal stress, disrupt species composition, and alter ecological function. In Auckland, thermal pollution has emerged as an important management concern, particularly in urban and unshaded rural streams, where ongoing land development and catchment modification increase the risk of exceedance of ecological temperature thresholds.

This report presents a regional assessment of stream water temperature across 39 sites in the Auckland region, using automated high-frequency records (maximum 15-minute logging interval) to evaluate both acute (daily maximum) and chronic (5-day Cox-Rutherford Index, CRI) thermal attributes. Temperature bands were assigned following the framework proposed by Clapcott et al. (2015), adapted to align with the National Policy Statement for Freshwater Management 2020 – Amended October 2024 (NPS-FM) additional attribute requirements. Data were analysed for two 5-year periods representing baseline (2013-2017) and current state (2020-2024) conditions.

Overall, the analysis found that most Auckland streams fall within B and C bands, indicating moderate thermal stress on aquatic ecosystems. Native forest streams were the coolest, while urban streams – particularly those with limited riparian shading and modified or concrete-lined channels –were consistently the warmest. Four urban sites exceeded the proposed regional bottom line (Band D), representing conditions of significant ecological stress. Of the 39 sites assessed, eight changed bands between baseline and current states, with the majority at a worse band for current state, representing warmer temperatures.

Long-term 5-year rolling medians showed that most streams have relatively stable temperature regimes, remaining within one band over the observation period. However, increasing trends in both daily maxima and CRI were observed in several rural (Wairoa, Rangitōpuni, Hōteo, Alexandra) and urban streams (Awaruku and Lucas). In contrast, Vaughan Stream exhibited decreasing temperature trends coinciding with improvements in turbidity and metal concentrations, likely reflecting the effectiveness of integrated Water Sensitive Design (WSD) and riparian restoration implemented as part of the Long Bay urban development.

Multiple linear regression analyses confirmed that reach-scale riparian shading and channel modification are significant predictors of stream temperature, together explaining up to half of the observed variation across sites. Streams with high (>90%) shading and natural channels exhibited significantly lower daily maxima and CRI values compared with modified or unshaded reaches.

A comparison between continuous and discrete temperature datasets revealed that discrete monthly measurements underestimate the hottest day of the year by an average of 3°C, particularly missing short-term heat extremes. As a result, discrete data tended to overestimate stream condition, with
State of stream temperature in Auckland. NPS -FM assessment, monitoring data 2020 -2024 2
more sites classified in higher (better) NPS-FM bands compared with continuous datasets. These findings support the use of continuous (or high-frequency) monitoring of temperature for robust thermal attribute assessment in Auckland’s rivers and streams.

In summary, this assessment demonstrates that stream thermal regimes in the Auckland region are closely linked to land use, shading, and channel condition. Maintaining and restoring riparian vegetation, reducing channel modification, and embedding Wate Sensitive Design (WSD) are essential for mitigating thermal pollution and protecting freshwater ecosystem health under ongoing urban growth and rural production and climate warming.

This report builds on the 2023 baseline state assessment (Auckland Council, 2023) by expanding data coverage and improving confidence in state evaluations. Additional years of monitoring data have enabled more sites to meet minimum data requirements and reduced key data gaps identified in the previous assessment, allowing longer-term changes in in-stream temperature to be evaluated. The analysis is further extended by examining potential drivers of in-stream temperature, including shade cover and channel modification, and by comparing discrete measurements with high-frequency monitoring data. Together, these improvements deliver a stronger and more comprehensive baseline for evaluating future change, while ensuring consistency with previous assessments.

Auckland Council technical report, TR2026/1

January 2026