The investigation into the wastewater spill that filled Lake Taupō with sewerage has concluded with no plans to prosecute.

A five-month investigation took place since the incident in July, with the Waikato Regional Council deciding it would not initiate a prosecution.

About 800,000 litres of wastewater, including raw sewage, was discharged directly into the lake on July 2 and 3.

A burst underground water main caused a footpath to collapse on to a sewerage pipe, resulting in it breaking and discharging wastewater to the lake.

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The site of the water main break and wastewater spill on the Taupō lakefront. Photo / File
The site of the water main break and wastewater spill on the Taupō lakefront. Photo / File

There was a further discharge of wastewater to the lake on July 16 while the Taupō District Council was working to fix the problem.

Waikato Regional Council's investigations manager Patrick Lynch said there had been a thorough and independent investigation into the July incidents.

"There is no doubt this was, at the time, a terrible incident with far-reaching impacts on iwi, residents, water users and visitors," said Lynch.

Repair work at the site of the wastewater spill on Lake Terrace, Taupō. Photo / File
Repair work at the site of the wastewater spill on Lake Terrace, Taupō. Photo / File

"The purpose of the Resource Management Act (RMA) is very clear – that is to promote sustainable management of natural and physical resources, and this incident clearly goes completely against that purpose.

"However, the RMA does allow for 'accidents' in situations where the incident was not foreseeable, was beyond anyone's control and was adequately mitigated. In our view, that is the situation here.

He said due to the "unique set of circumstances" it was not appropriate for anyone to face formal enforcement action.