Executive summary

Te Waihē / Mahurangi Harbour and its surrounding catchment is characterised by steep hill country, stunning harbour views and numerous small freshwater streams and harbour inlets. Located on the north-eastern coast of the Auckland region, long-term monitoring reflects sedimentation, with ecological shifts in intertidal areas, including more mud-tolerant taxa and fewer sand-dependant species. Sediment source tracking identifies erosion from pastoral land as a key contributor. Reducing erosion from rural land is therefore essential to protecting the environmental health and cultural values of the Te Waihē / Mahurangi Harbour.

The Mahurangi Land Restoration Programme (MLRP) was a $6.15 million programme funded by the Ministry for the Environment (MFE) Manatū mō te Taiao that was initiated by Auckland Council in partnership with the Ngāti Manuhiri Settlement Trust in 2020. The goal of the MLRP was to increase the mauri of the Te Waihē / Mahurangi Harbour by achieving measurable reductions in human-induced sediment loss from rural land, primarily through supporting landowners to retire and plant riparian margins, wetlands and erosion-prone hill country. Most of the waterways that are retired as part of the programme were small, headwater, tributaries including springs and seeps. The MLRP builds on over 20 years of land restoration in the catchment, commencing with the Mahurangi Action Plan in 2004.

This report presents findings from a three-year, space-for-time stream monitoring programme (2023-2025). Monitoring was designed to assessing whether riparian restoration (fencing and native planting) measurably reduced fine sediment levels and improved ecological health in headwater streams. Nine permanently flowing headwater streams were repeatedly monitored. Streams represented a gradient of riparian habitat quality and stock exclusion. Each stream was assigned to one of three riparian treatment groups: 1) open pasture (unrestored), 2) fenced – planted native scrub (restored), and 3) fenced – mature native forest (reference). Indicators include deposited fine sediment, turbidity, macroinvertebrate indices, the taxonomically independent community index (TICI), and reach-scale habitat quality assessed using the Rapid Habitat Assessment (RHA) method. The role of catchment-scale characteristics, including catchment size, native forest cover and stock exclusion, were also investigated.

Results showed a clear gradient in reach-scale habitat quality across the sites, being highest in forested reference streams, intermediate in restored streams and lowest in open pasture streams. Reach-scale habitat quality and the degree of stock exclusion in the catchment were the strongest predictors of fine sediment levels in streams.

Restored streams contained approximately 32% less fine sediment than open pasture streams. Inorganic mud (< 63 μm) accounted for approximately 67% of the total sediment in open pasture streams, compared to 29% in restored streams and 37% in forested streams. High frequency turbidity monitoring showed similar patterns.

Ecological health indicators – including the Macroinvertebrate Community Index (MCI), Quantitative Macroinvertebrate Community Index (QMCI), pollution sensitive mayflies, stoneflies and caddisflies (EPT) and TICI – were also positively correlated to reach-scale habitat quality (RHA) and key catchment characteristics (native bush cover, the degree of stock exclusion and catchment size). There were key differences in sensitive EPT taxa between riparian treatment groups. All three groups of EPT taxa were present in reference forested streams, mayflies and caddisflies were present in restored streams and only a few caddisfly taxa were present in open pasture streams.

Native freshwater species, including kākahi (freshwater mussel; Echyridella menziesii), giant kōkopu (Galaxias argenteus), and the regionally rare redfin bully (Gobiomorphus huttoni), were detection through environmental DNA analysis (eDNA). These species were predominantly found in forested and restored headwater streams, highlighting the importance of headwater streams as habitat for native fauna.

Overall, this study demonstrates that riparian restoration, through stock exclusion and native planting, is likely to be reducing fine sediment and improving ecological health in headwater streams. If left unprotected, headwater streams have the potential to deliver significant amounts of fine sediment cumulatively across the Mahurangi catchment. When protected, sediment levels reduce and the diversity of native species increases. The findings support prioritising headwater stream restoration as an effective strategy for improving catchment-wide outcomes.

Future restoration should prioritise headwater reaches as high‑value intervention points, using eco‑sourced native plant mixes, including 20% to 25% long‑lived species, supported by sustained pest and weed control. Connecting restored areas to create continuous riparian corridors, linked to native forest remnants, will enhance recolonisation and ecological recovery.

A catchment‑wide approach remains essential. Riparian management is most effective when combined with other mitigation approaches, such as spaced planting or afforestation of gullies and erosion‑prone slopes, wetland enhancement, good soil management practices to reduce bare ground and compaction, and the use of vegetated filter strips and small detainment bunds to intercept runoff. Importantly, increasing catchment-wide forest cover and forest edge density, is expected to moderate peak flows, a primary driver of erosion and sediment mobilisation.

Ongoing monitoring should include periodic assessments of restored streams across the Mahurangi catchment using sediment, ecological and cultural indicators, alongside targeted monitoring of peak sediment-loss events. The degree of forest cover and any changes in forest edge extent, should also be documented.

Central to the long-term success of catchment restoration programmes is maintaining a strong partnership with mana whenua, landowners and the wider community. Recognising that ecological restoration is people powered and unfolds over decades.

Auckland Council technical report, TR2026/6.

Auckland Council, June 2026.