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Northern Wairoa River

The Waiotu and Whakapara rivers form the headwaters of the Wairua River. They begin in the eastern ranges between Whangarei and Kawakawa and join in the northern part of the Hikurangi Swamp. Both river catchments quickly turn from native forest to pastoral land, which is the dominant land use and both cut through hard sediments formed from faulted greywacke, along a relatively low gradient.

The Mangahahuru Stream joins the Wairua River in the southern part of the Hikurangi Swamp. The upper reaches of the Mangahahuru are dominated by pine forestry with the lower reaches pastoral land. The productive Hikurangi Swamp is dominated by dairy farms and has an extensive drainage scheme. A dairy factory and the Hikurangi oxidation ponds both have consent to discharge waste into the Wairua River catchment.

The Wairua and Mangakahia Rivers join to form the Wairoa River just north of Tangiteroria. The Mangakahia River is 104 km long and has a total catchment area of 810 square km. The upper reaches of the Mangakahia are a mixture of plantation forestry and sheep and beef farming.
The Opouteke River is a major tributary of the Mangakahia River and is 31 km long. It joins the Mangakahia River at Pakotai and drains a predominately pine forest catchment.

The Manganui River begins along the western fringes of the Marertu Forest and flows northwest to eventually join the Wairoa River to the east of Dargaville. The river is 91 km long and drains a significant catchment area.

Another major tributary of the Wairoa River is the Kaihu River which originates in native forest to the west of Trounson Kauri Park and flows south, where it joins the Wairoa to the east of Dargaville. The catchment is a mix of exotic and native forest and agricultural land use.

Water quality in the Wairoa River is degraded largely due to pastoral farming which dominates this catchment. Water quality monitoring sites surrounded by pastoral land use (Mangere, Manganui and Whakapara) typically have elevated nutrient levels, particularly phosphorus and turbidity. Sites situated in native and/or pine forest (upper Mangakahia and Opouteke) have lower nutrient levels and generally good clarity (prior to forest harvesting).

Sites 12

Monitored sites in the Northern Wairoa River catchment

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Scientific Indicators
Scientific data for this catchment

This dashboard shows information on the data collected by the regional councils for water quality indicators, analysed as state and trend.

The state for the catchment is represented by the median concentration for the parameter across all sites within the catchment and then compares that value to the quartiles for all monitored sites in New Zealand.

Click on the parameters state icons to compare this catchment with others in the region.

  • State

    State shows how the median of samples from this site compares to other sites

  • Trend

    Trend shows how the quality of water is changing over time. Depending on the sampling history duration, five and ten year timescales are available:

Can I trust this data?

The Cawthron Institute has worked alongside regional councils to verify the processes and methods used for data collection, laboratory analysis of samples collected and the statistical analysis and interpretation of the results presented.

If all Cawthron ticks are green, then you can trust this data. However, if one or more ticks are orange, then conclusions should be treated with some caution.

For more details on each tick, see our 'Can I Trust This Data?' Factsheet.

Can I trust the data?
Data Collection

All samples were collected using approved field protocols and have been analysed in accredited laboratories. Therefore the data shown here has been collected and analysed following best practice.


All samples were collected using approved field protocols and have been analysed in accredited laboratories.

Trend - Adjustment for flow

This site is not sampled during high flows so the data shown only represents base flow conditions. Proposed national guidelines recommend sampling during both low and high flows to get a full picture of water quality at the site.


E. coli



Black disc





Total Nitrogen


Total Oxidised Nitrogen


Ammoniacal Nitrogen



Dissolved Reactive Phosphorus


Total Phosphorus

Download Data

.ZIP file of indicator readings for all sites in New Zealand

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