The general location on the Waikato River of the wastewater incident in February 2018.

Discharging 1.782 million litres of wastewater including human sewage into the Waikato River has seen the Hamilton City Council convicted and fined $54,000.

The discharge from its central city wastewater pumping station on Anzac Parade occurred over a 19-hour period on February 20, 2018.

A city council engineer was alerted to a potential problem through routine systems monitoring which led to the discovery of the overflow.

According to the sentencing notes, that engineer noticed an anomaly within the station's trend data, which appeared to have flatlined.

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This indicated there was no pumping activity at that station. The engineer then discovered the wastewater was discharging into the Waikato River and immediately over-rode the sensor equipment and manually operated the pumps, which instantly stopped the overflow discharge.

Hamilton City Council reported the discharge to the Waikato Regional Council.

It was found that human error had resulted in both the primary and back up equipment, designed to operate the pumps and alarms, being inadvertently made inoperable.

An internal investigation by the Hamilton City Council found two faults: the wastewater pumps had not been signalled to start, and high-level alarms were not triggered.

Both would normally occur in response to increasing levels of wastewater in the pump station's well.

Hamilton City Council waters manager Maire Porter said their investigation discovered the pump's span setting - the sensor system used to measure wastewater levels in the underground holding wells - had been changed.

Normally, when the wastewater reached a certain level, the sensors triggered the pump to start pumping, she said.

"The pump was meant to come on normally when the wastewater level was 2.2 metres below the top of the well. That band setting had been changed from 2.2m to 0.22m."

That meant the level of wastewater was at an extremely high level before the pumps started.

Turning off the wastewater station's pumps required access to its computer software. This required a password, but not a password that was user specific. Anyone with proximity to one of the few computer terminals within the greater Hamilton city area that is connected with this software could have done this.

The Council was unable to find out who changed that setting. This had triggered some of the corrective actions it has now put in place, Porter said.

"So we can have better transparency on who is making changes and when they are making those changes."

Waikato Regional Council subsequently took a prosecution against the council under the Resource Management Act.

District Court Judge Melinda Dickey passed sentence on Hamilton City Council last week, stating that the council was "careless" because of the failure of its failsafe systems.

"I acknowledge that both failures occurred as a result of human error."

Judge Dickey noted that "any overflow would result in a direct discharge of untreated human effluent into a river of significance" and that the river "required a more robust set of protections".

"I acknowledge that the council, upon discovering the issues, took immediate steps to remedy them, however the fact that improvements have been able to be made to its systems and infrastructure tells me that the system was somewhat vulnerable."

Judge Dickey noted the council had entered a guilty plea at the first reasonable opportunity, had shown considerable remorse, and had identified several corrective actions to prevent similar discharge events recurring.

As part of its redress actions the council agreed to undertake education and awareness programmes, carry out riparian planting and implement a new internship programme.

It also participated in two positive and productive restorative justice meetings. Representatives from Waikato-Tainui and Te Haa o Te Whenua o Kirikiriroa (THaWK), representing local mana whenua, attended.

The council has also undertaken several corrective actions to prevent further discharges.

These include further training for maintenance staff, updating its processes to prevent maintenance staff changing the wastewater system's critical control points, programming in 'pump not running alarms' into critical sewage pumping stations and restricting access to its computer software system.

Porter said those actions have been undertaken on the council's other pump stations as well.

"We've definitely used this as a learning opportunity and have applied all of the learnings across all out pump stations as well as our other infrastructure."

When asked if the council was confident it had the measures in place to prevent another discharge from occurring, Porter said:

"In terms of what happened at Bridge Street, yes, but it is a very challenging environment that we work in terms of collecting 50,000 cubic metres of wastewater every day and transferring it out to our treatment plant. There's always the unexpected that can occur with wastewater systems."