Tomahawk Lagoon Dunedin

Tomahawk Lagoon. Photo: Stephen Jaquiery


In 2017, in response to concerns about water quality, our staff began to work with the local community to scope lake restoration works for Tomahawk Lagoon.

Water quality was found to be degrading with elevated levels of sediment, E. coli, nitrates and phosphates found within the lagoon and Lagoon Creek (a contributing waterway).

Cyanobacteria is found within the lagoon regularly in the summer months. The community also raised other concerns to do with water levels and sediment.

With the establishment of the Environmental Implementation Team in 2021, together with mana whenua, stakeholders, and the community we agreed on 3 key goals: an ecological assessment of the catchment, the support and formation of a catchment group and the establishment of a permanent water quality monitoring site for the lagoon.

About Tomohaka / Tomahawk Lagoon

Testing water quality of Tomahawk Lagoon

Associate Professor Marc Schallenberg examines a sediment core sample from Tomahawk Lagoon.


Tomahawk Lagoon is located at the southern end of the Otago Peninsula. It consists of two shallow brackish water lagoons which are joined by a narrow channel and weir/gate structure and share a common sea outlet.

Tomahawk Lagoon is a wildlife refuge of 33 hectares that is managed by the Department of Conservation which contains the threatened plant species Isolepis basilaris on the margin of the lagoon. The area is important ecologically as it is defined as a marsh and less than 15% of original marshes remain in Otago.

The lagoon is a regionally significant wetland habitat for waterfowl and waders with a variety of bird species present and is part of a chain of feeding habitats along the coast used by migrating birds. Tomahawk Lagoon is also important habitat for native fish and eels.

Citizen Science water quality monitoring is undertaken by ECOTAGO monthly. ECOTAGO are a community group who are working to test and improve water quality within the Tomahawk Lagoon catchment.

Their results indicate that turbidity, E. coli, nitrates and phosphate levels are all exceeding national guidelines. Cyanobacteria is also regularly found within the lagoon over the summer months which is a toxic bacterium which can be harmful to humans and animals.

Aaron Heimann (Former ORC Catchment Advisor) and Andrew Innes from ECOTAGO undertakes fish sampling with gee minnow nets at Tomahawk Lagoon.

The project and consultation process

ORC has dedicated $260,000 over three years (2022-2025) to enhance the Tomahawk Lagoon Catchment through community action to contribute to a healthy ecosystem for all to enjoy. 

In 2018, two workshops were held with the community which identified community goals and values for the Tomahawk Lagoon catchment, and it identified potential projects for the catchment. 

In 2021, a consultation process was undertaken with the community, mana whenua, and key stakeholders to vote on the highest priority actions to be implemented for the catchment.

The three priority actions that were identified by the community, mana whenua and key stakeholders in 2021 were:

  1. Catchment group formed and supported
    • Joining of agencies and the community. Establish a ‘Friends of Tomahawk Lagoon’ with members of the community and stakeholder representatives
  2. Water Quality data
    • Have a permanent water quality monitoring site installed to establish baseline data and ensure mahika kai safety.
  3. Ecological Assessment 
    • Investigate the balance between the needs of human interaction with the lagoon and wildlife (hydrological function, ecology, wildlife, walking tracks, flood hazard, core sample). Include assessment of what the limits are for the system in this catchment (tipping point). What are the key stressors and how resilient is the catchment. What actions do we need to undertake to make the catchment more resilient.

The three priority actions, along with the collective vision and values for Tomahawk Lagoon, were compiled in the Tomahawk Lagoon Outline Management Plan. 

This outline management plan was then approved by council for delivery.

ORC has been supporting the community and the Otago Catchment Community Inc in the formation of the Tomahawk Lagoon catchment group. Steve, Dean and Andrew from Tomahawk lagoon citizens science team have been working hard to develop this group and encourage other community members to join.  

View Tomahawk Lagoon Citizen Science Team results

Progress so far on the three priority actions

Catchment group formed and supported

This work is being led by the community and Otago Catchment Community. ORC continues to support community members in the formation of a ‘Tomahawk Lagoon Catchment Group’.

Ecological Assessment

ORC has 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.

Water Quality Data

ORC has established a monitoring site as part of our State of the Environment programme (August 2022) in the deepest point of the upper lagoon. The site name is Tomahawk Lagoon at Tomahawk Mid, and we perform monthly sampling for various water quality parameters (e.g., nutrients, chlorophyl-a, bacteria levels, and water clarity) and physico-chemical parameters (water temperature, pH, dissolved oxygen, conductivity, etc).

What else is happening?

ORC has contracted 4Sight Consulting to design and deliver a stormwater education programme at Tomahawk Lagoon in early 2024. The aim of this programme is to do some shared learning around what stormwater is, where it goes, what this means for Tomahawk Lagoon, and how we can all do our part to minimise our impact on stormwater water quality.

Next steps

Now that we have received the report on the ecological assessment of Tomahawk Lagoon, ORC has begun drafting a plan for implementation of some of the recommended actions.

Get in touch

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

Get involved with your community

If you would like to get involved with the local community on this project, follow them on social media:

Open Day at Tomahawk Lagoon photo gallery