A major investigation of the Tauranga Harbour floor is under way as Western Bay of Plenty District Council explores long-term options for managing treated wastewater from Katikati.

Specialist contractors have begun surveying the harbour between Katikati and Matakana Island, collecting samples and assessing ground conditions as part of planning for a potential replacement of the ageing wastewater pipeline that crosses the harbour.

The work is part of the council's Katikati Wastewater Disposal Future Directions project, which is being guided by a project control group and Te Ohu Waiora, comprising representatives from tangata whenua, the council and external stakeholders.

Western Bay of Plenty District Council director of water services Coral-Lee Ertel said the investigations were a key step in determining a sustainable wastewater solution for the future.

"We're focused on implementing a sustainable, long-term and culturally acceptable solution that protects our harbour, meets new wastewater standards and reflects the values of our community and tangata whenua," Ertel said.

The Katikati Wastewater Treatment Plant currently discharges treated wastewater through an ocean outfall pipeline that runs across Tauranga Harbour, through Matakana Island and out to sea.

The council said the harbour section of the pipeline had experienced several breaks during the past year, highlighting the need for a long-term replacement strategy.

The plant's discharge consent expires in 2038, and the council is required to confirm its preferred wastewater disposal option by the end of this year.

Following completion of the Future Directions Report in 2025, the council identified retention of an ocean discharge as the preferred long-term option, with higher treatment standards and a longer outfall pipe. The early replacement of the ageing harbour crossing section of the pipeline was also identified as a priority project.

The current survey focuses specifically on the option of replacing that harbour section.

Information gathered from the investigation will help determine whether ground conditions are suitable for installing a replacement pipeline, support engineering design work and inform future resource consent applications.

Brown Bros Drilling was selected to carry out the survey following a procurement process.

The investigation includes 14 sites along a proposed replacement route located north of the existing pipeline.

At each site, crews will collect sediment and soil samples and carry out specialised testing to assess the strength and stability of the harbour floor to depths of up to 15 metres.

Sediment samples from five locations will also be tested for environmental contaminants as part of an ecological assessment.

Katikati Wastewater Disposal - map of current system. Image / Western Bay of Plenty Regional Council

The work is expected to take up to two weeks to complete, depending on weather and tidal conditions. The council said no disruption to residents or harbour users is anticipated.

Ertel said the investigations would provide critical information about the harbour environment and future construction requirements.

"With the current outfall pipeline nearing the end of its operational life and experiencing multiple leaks, we're working to address both the current infrastructure issues and future regulatory requirements," she said.

"Understanding the ecological conditions and the nature of the harbour floor is essential and the results from this survey will help us determine the best construction method to ensure a replacement pipeline can be safely and responsibly constructed, if that becomes the preferred solution."

The council said the community would have an opportunity to provide feedback on the preferred wastewater disposal option once technical and environmental assessments had been completed.

Further information on the project is available via the council's Katikati Wastewater Disposal Future Directions project page.