Water quality in the Manukau

By John Duncan

The Research and Evaluation Unit of Auckland Council have recently published the latest (2018) data for water quality in the Manukau[i].  Coastal and estuarine water quality has been monitored at sites across the Auckland region since 1987. Long-term monitoring is important for identifying trends in water quality parameters that are naturally highly variable between seasons and years, allowing better understanding of human induced change. It is important, whatever restrictions COVID or other impacts may have on budgets, that these long-term series on chemistry and ecology are continued.

The trends from 2007 to 2016 were that water quality was generally improving across the Auckland region. The overall results of the 2016-18 data analysis are broadly consistent with this, but water quality at the Clarks Beach site was poorer.   (When the report author refers to ‘Clarks Beach’, the sampling point seems to be in the channel between Torkar Point and Matakawau – in the author’s own words, ‘The Clarks Beach site is located at the mouth of the Waiuku and Taihiki Estuaries which include the small urban area of Waiuku town and high intensity rural land use including dairy, beef farming, and extensive market gardening.’) Many parameters still exceeded water quality guidelines in the Manukau – the key parameters of concern are ammoniacal nitrogen, total oxidised nitrogen, and soluble reactive phosphorus – effectively, fertilisers!

The state of the marine water quality across the Auckland region is strongly related to median salinity, which is indicative of the proximity of a coastal or estuarine site to freshwater inputs, and the extent of flushing and dilution at the site. Several sites within the Manukau Harbour (including Waiuku Town Basin and Clarks Beach) had poorer water quality than expected for the salinity of the site. The report author notes that more detailed investigation of the causes of this poorer-than-expected water quality at Manukau Harbour sites (in light of the salinity at the sites) should be enabled through analysis of the upcoming hydrodynamic and water quality model.

This model was commissioned by Watercare in 2016, with an expected delivery date in 2019. The Manukau Harbour Forum Symposium in August 2019 was told it was expected to be delivered by Christmas that year. The revised estimated delivery date in February 2020 (note – before any COVID interruptions) was now ‘the second half of 2020’. It is difficult to believe that conscientious project managers should have still been aware, four months before their expected delivery date, that the delivery was still a year away, and has to raise questions whether Watercare gave this low priority, or is looking for a way around the findings that the model is giving.

For much more detail on the water quality findings than this brief article can tell you, see the full report at

[i] Ingley, R (2019). Coastal and estuarine water quality: 2018 annual data report. Auckland Council technical report, TR2019/027