Mangawhai Harbour Water Quality Project

Mangawhai Harbour Water Quality Project

Mangawhai Harbour Water Quality Project (NIWA Report)

Why was the study undertaken?

The Mangawhai Harbour Water Quality Advisory Panel emerged from a review of the Mangawhai Community Wastewater Scheme, which expressed concern at the lack of any comprehensive ongoing monitoring of water quality in the Mangawhai harbour. Discussion between members of the  Advisory Panel, Kaipara District Council and Northland Regional Council staff, led to a shift in focus from the harbour itself to inflows into the harbour. The reasoning was that the largest contributor of contaminants would be the harbour’s two sub-catchments rather than the much more limited harbour edge. The primary objective of the project was to characterise and compare water quality from the different sub-catchments. The key questions it was hoped the study would answer were:

  • How different are the two major inflows into the harbour from the south and the north sub-catchments for a range of physical, chemical, and biological attributes?
  • Within the two major sub-catchments how different are the inflows from their sub-catchments and do these reflect land-use and ground cover?
  • How do the inflows from disturbed/developed catchments compare with an undisturbed native forested catchment?

The proposed testing would include the physical attributes of salinity, water temperature, dissolved oxygen, turbidity and visual clarity, biological attributes of total coliforms and E. coli, and chemical attributes of ammoniacal nitrogen, total nitrogen, and total phosphorus. Sampling would be carried out at approximately fortnightly intervals around low water as measured at Back Bay, with additional sampling after significant rainfall events. The six sampling sites as shown in the attached map are; Devich Road bridge, Insley Street bridge north and south, Molesworth causeway, Henry’s Bridge on Cove Road (Tara Creek), and the Forest Stream off King Road.


Who undertook the work?

The voluntary Advisory Panel members were supplemented by further skilled community members to establish two sampling teams of 3, undertaking alternative sampling days, with the two team leaders attending all sampling days to ensure consistent sampling practice. Training and equipment were provided by NRC and administrative support by KDC. The only direct costs were laboratory analysis of the water samples, input from Te Uri O Hau, and analysis and reporting of the full data set by NIWA. These costs were covered by a grant from the MELA fund and contribution from NRC.

What were the key findings?

The full year data set included 25 programmed fortnightly samples and 2 samples dictated by high rainfall events. A single sample was discarded as it reached the laboratory outside the required 24 hour window. The NIWA analysis examined;

  • how water quality changed from the upper sub-catchments to where it enters the harbour,
  • how water quality differed between pastoral and forested sub-catchments,
  •  how water quality was affected by rainfall, and
  • The characteristics of each of the sampling sites.

The two extremes of water quality within the study are the Forest Stream for which the median values of all variables were within the ANZECC (2000) guidelines for lowland streams, compared with Tara Creek at Henry’s Bridge which had the highest values for total P, NH4-N, E. coli, and turbidity.

Total phosphorus (TP), Total Nitrogen (TN), and Ammonium. The Forest Stream had by far the lowest levels of these three nutrients with median levels of 0.04, 0.24, and 0.008 mg/L respectively. Median levels at the other 5 sites ranged between 0.6 and 0.8 for phosphorus, 0.46 and 0.57 for nitrogen and 0.03 and 0.06 for ammonium. These nutrients are key indicators of water quality in estuaries and can arise from both natural sources and human activities such as sewerage leakage, dairy shed effluent, and fertiliser runoff. At the sampling sites at Tara Creek and Devich Road Bridge levels of 3 to 6 times the ANZECC guidelines for nitrogen and ammonium were recorded.

Escherichia coli (E. coli). Even the Forest Stream showed high E. coli levels after heavy rain, probably from birds, pigs, and other wildlife in the catchment (median 115 MPN/100 mL, range 10 – 65000). However, the highest levels in this stream were 3 times less than maximum values for any other site. The Tara Creek medium value was 590 MPN/100 mL with a range of 100 – 21000, and at Devich Road Bridge 215 with a range of 68 – 20000. High levels of E. coli during no rainfall found in Tara Creek suggest a point source of faecal matter into the stream with a similar situation at the north side of the Insley Road bridge which is fed from the stream passing along the western edge of the domain. The E. coli analysis did not separate animal and human components.

Turbidity and clarity. High rainfall events as expected caused high turbidity (actual sediment in the water sample) and low clarity levels at all sampling sites but once again the Forest Stream was much lower and cleared much more quickly than any other site (median turbidity 3.25 NTU, range 1.1 – 55) At Tara Creek comparable values were 9.2 and 3.7 – 180. A very high correlation was found between turbidity and E. coli levels (r = 0.82) suggesting overland flow is the most important contributor of contaminants.

Salinity and dissolved oxygen. As expected salinity was very low except at the Insley Street and Molesworth Drive Causeway sites where tidal influences were greatest, while dissolved oxygen was highest in the Forest Stream.

The full NIWA report with detailed data from all sites is available on the KDC website.

Where to from here?

The report has raised a number of issues which require clarification and has suggested strategic directions which would clearly benefit the water quality of our harbour. Further targeted sampling is proposed to;

  • Identify point sources of E. coli contamination above Tara Creek and above the north Insley Street Causeway sampling site,
  • Separate human E. coli from that of other animals in future samples, and
  •  Determine if turbidity combined with flow data could be used as a low cost indicator of E. coli export into the harbour.

The study clearly points to where restoration planting and stream edge retirement might make the greatest contribution to reducing nutrients and bacterial contamination entering streams from overland flows during high rainfall events. We now much better understand the nature and general sources of contaminants entering our harbour, clearly the first step in developing a community driven strategy tackling the issue.



Forest Stream at normal flow.


Forest Stream water sampling following a major rainfall event.

If you would like to contact the water quality advisory panel please email


Online requests 

Kaipara District Council
0800 727 059 (24 Hours)
Phone: +64 9 439 7059


Office hours: 8 am to 4.30 pm Monday to Friday

Address: 42 Hokianga Road, Dargaville 0310
Address: Unit 6, The Hub, 6 Molesworth Drive, Mangawhai 0505

Mail: Private Bag 1001, Dargaville 0340


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