Nelson Wastewater Treatment Plant

Nelson Wastewater Treatment Plant – birding highlights

Construction of the Tasman Bay outfall was completed in 1970, with the right to discharge wastewater into the adjacent Tasman Bay. Piped some 300 metres out to sea, in winter up to 50 North Giant Petrel, and small groups and rafts of several hundred Snares Cape Petrel (500+ in October 1982) were recorded feeding on this outfall. The current 26 hectare oxidation pond was constructed in 1979. In 1996 the oxidation pond was sub-divided into two interlinked ponds, to improve the discharge quality into Tasman Bay – much, no doubt, to the petrels’ dismay. Wastewater from Nelson City is piped from Neale Park pump station to the NWTP. Before entering the oxidation pond the wastewater goes through several treatments including the removal of solids and sludge, and bacterial treatment of sewage. In a natural biological process, the first oxidation pond (facultative) further breaks down the sewage, whilst the second pond (maturation) removes faecal coliforms. In 2010, two 7 hectare planted wetland ponds were constructed so effluent could be further treated before going through the outfall pipe. The plantings included two strips with sedges on each pond, which initially thrived, but were to die off completely – Pukeko and Black Swan copping the blame for this.

The building of the oxidation pond, and then the wetlands, has had a significant impact on bird life. Construction of the road to the plant and tidal flood gate cut off the tidal flow to the head of the Nelson Haven estuary and saltmarsh, where breeding Banded Rails and Bitterns occurred. Years of sedimentation have transformed this coastal saltmarsh to rough grassland and scrub.  However, the established oxidation pond and treatment wetlands have become a haven for wildfowl, and overall have greatly enhanced the biodiversity of the area. Prior to the construction of the wetlands, there were informal ephemeral scrapes and ponds, on which the godwits would roost and rare waders, such as Whimbrel, Ruff, Black Stilt, Marsh Sandpiper, NZ Dotterel all made an appearance. Pied Stilt nested on the wetlands before the ponds were inundated (19 nests) and continue to make use of the shallower water where the planted strips were to feed. Including what is now the wetlands area both before and after their construction, the NWTP has been host to at least 69 species.

The first Nelson records of NZ Scaup were on the oxidation pond in 2008 with 3 birds. This became 24 in 2011 with the wetlands up and going, to 400 in 2016, and since 2021 counts of over 1,000 are not uncommon. This makes NWTP a Wetland of International Importance as it supports at least 1% (closer to 3%) of the world population. NZ Dabchick, first seen on the oxidation pond in 2010, has turned into an autumn/winter annual visitor with as many as 6 birds. Coots pop in from time to time, and a residual Marsh Crake or two, from an influx in 2013, are also occasionally recorded. Australasian Crested Grebe was recorded on the treatment wetlands in 2023, the first record for 50 years in the region There has also been a nice array of rarities, including single Australian White-eyed Duck/Hardhead, Northern Shoveler,  Australasian Little Grebe and Red-necked Phalarope, plus up to 5 White-winged Black Tern. Paradise Shelduck peak between December and May, predominantly on the bunds of the oxidation pond, with much aerial traffic between there and the paddocks to the north of the flats. Scaup are generally more numerous over winter, though summer numbers are still extremely healthy. Grey Teal tends to show larger numbers between November and April, whilst Australasian Shoveler do so between February and August. However, like Scaup, both of these species are highly mobile, and can come and go and confound any perceived population pattern.  Black Swan numbers are more or less the same throughout the year. Their numbers increased from a high of about 50 to over 200 recorded once the wetlands were built (in 2011). Active discouragement – including culling – has seen these numbers reduced to their pre-wetlands levels. Scaup have been breeding on the wetlands since 2012. Grey Teal and Australasian Shoveler have also thrived here, it being a major stronghold for them both in the region, and breed in small numbers. Mallard, Paradise Shelduck and Black Swan also breed around the treatment wetlands, with nesting for the latter as early as June. Despite yearly encouragement from the side-lines, NZ Dabchick have as yet to nest here.

Tight-knit feeding flocks of Red-billed Gulls of 100 to 400 birds are present on the wetlands ponds, generally between January and April, often with small numbers of Black-billed Gulls amongst them. Flocks of hundreds of Welcome Swallows can appear at almost any time of the year (January to September), then disappear just as quickly to breed. Interestingly, Pied Shag is the only waterbird to have declined in numbers since the wetlands were opened. They have reduced from over 15 (with a high of 33) to generally less than half that number. Small bands of SIPO are seen particularly after spring tides flying from the fields to the north of NWTP to the Haven, as are small numbers of White-faced Heron. VOC are also seen, and even the occasional small flight of godwits. A healthy population of Fernbird, approximately 20 territories (49 birds banded in 2021) has been monitored by OSNZ on the adjacent flats, spilling over onto the fringes of the ponds. Unfortunately, the 2022 floods inundated their habitat, severely reducing the population to about half its previous numbers. Cirl Buntings are about in small numbers, though sightings are not all that common. The odd New Zealand Pipit was to be seen on the wetlands bunds when mowed, though more common along the Boulder Bank and adjacent open sandflats. Flocks of finches including Lesser Redpoll can be seen along the bunds between the oxidation and wetland ponds over winter. One of the sounds of the summer (and almost year-round) is the song of the Skylark. Swamp Harriers are regularly seen quartering over the wetlands, most closely when young ducklings are on the water.