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Tomohaka / Tomahawk Lagoon
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Tomahawk Lagoon sits at the southern end of the Otago Peninsula and is made up of two shallow brackish lagoons connected by a narrow channel and a weir, with a shared outlet to the sea.
It is a wildlife refuge of 33 hectares managed by the Department of Conservation and home to the threatened plant species Isolepis basilaris . The area is ecologically important as it is defined as a marsh - less than 15% of Otago’s original marshes remain.
The lagoon is a key wetland for waterfowl, waders, and migratory birds, and also provides habitat for native fish and eels.
Citizen Science results indicate that turbidity, E. coli, nitrates and phosphate levels are all exceeding national guidelines. Cyanobacteria, which can be harmful to people and animals, often appears in summer.
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
In 2017, ORC began working with the local community to address water quality concerns at Tomahawk Lagoon.
We’ve committed $260,000 over three years (2022–2025) to support catchment restoration through community-led action. In 2018, two workshops were held with the community to identify community goals and values, and potential projects for the catchment. In 2021, consultation with the community, mana whenua, and key stakeholders helped prioritise actions to be implemented for the catchment, with the top three outlined in the Tomahawk Lagoon Management Plan.
In 2022, ORC set up a monitoring site (named: Tomahawk Lagoon – Tomahawk Mid) at the deepest point of the upper lagoon as part of our State of the Environment programme. Water quality (nutrients, chlorophyll-a, bacteria, clarity) and physical parameters like temperature, pH, dissolved oxygen, and conductivity are sampled on a monthly basis.
In 2023, an Ecological Assessment of Tomahawk Lagoon was completed by Cawthron Institute, to do a study into ecological health and processes that control water quality at the lagoon. It provids an understanding of restoration options that could be employed to improve water quality and safeguard the ecological health of the catchment. The final report delivered on 31 July 2023 was shared with mana whenua, community representatives, and key stakeholders with the below recommended actions focus on reducing contaminants:
- Consider zoning the catchment as a nutrient-sensitive area in ORC’s Land and Water Plan
- Fencing and riparian planting, plus forestry buffers
- Enhancing existing wetlands
This important work is being driven by the community and Otago Catchment Community, with ORC continue to provide support. A Tomahawk Lagoon Catchment Group was formed to lead the effort, and since then, many community days and events have been held—moving us closer to the shared vision:
“A thriving Tomahawk Lagoon catchment, where water quality and biodiversity are maintained and enhanced through community action, contributing to a healthy ecosystem for all to enjoy.”
Associate Professor Marc Schallenberg examines a sediment core sample from Tomahawk Lagoon
A pied stilt or white-headed stilt (Himantopus leucocephalus) is bending over and looking between its long legs, at Tomahawk Lagoon
A pied stilt or white-headed stilt (Himantopus leucocephalus) is moving in the strong wind, at Tomahawk Lagoon
A Marsh crake or Baillon's crake (Porzana pusilla), New Zealand endemic subspecies P. p. affinis is walking (foraging) in the reeds near water, at Tomahawk Lagoon
Tomahawk Lagoon Sep 2025. Photo: Chris Peacocke
Ecological Assessment
ORC contracted Cawthron Institute to undertake an Ecological Assessment of Tomahawk Lagoon. ORC contracted Cawthron Institute to undertake an Ecological Assessment of Tomahawk Lagoon.
The aim of the ecological assessment was to identify drivers for the current ecological status of the lake and to identify possible actions that the ORC, in collaboration with the community, mana whenua, and key stakeholders, could undertake to make the catchment more resilient. The ecological assessment consisted of an analysis of historical data, and a fieldwork component. The final report on the ecological assessment was submitted to ORC on 31 July 2023, which was then shared to mana whenua, community representatives, and key stakeholders.
Recommended management actions focussed on reducing contaminants in the lagoons. These included:
- Exploring the possibility of zoning the catchment as a nutrient-sensitive zone in the Otago Regional Council Land / Water Plan
- Undertaking fencing and riparian planting; buffers in forestry areas
- Enhancing existing wetlands in the catchment
Tomahawk Lagoon Community Day — April 2024
Open day - July 2023
Open day held at Tomahawk Lagoon.
Associate Professor Marc Schallenberg demonstrating the scientific value of sediment core samples at an Open Day event at Tomohaka / Tomahawk Lagoon.
local school pupils getting involved in the science at Tomahawk Lagoon.
local school pupils getting involved in the science at Tomahawk Lagoon.
ORC has developed an Implementation Plan with the community.
The Vision is a thriving Tomahawk Lagoon catchment, where water quality and biodiversity are maintained and enhanced through community action to contribute to a healthy ecosystem for all to enjoy.
ORC has more funding available to help landowners enhance habitat on their properties, particularly wetlands and alongside creeks.
More stormwater education and signage is being developed.
If you would like to get involved with the local community on this project, follow them on social media:
If you would like to know more about this project or have any other questions about the work at Tomahawk Lagoon, you can contact our customer services team:
Phone: 0800 474 082
Email: customerservices@orc.govt.nz
