Wellington Water could face prosecution after two Olympic sized swimming pools worth of wastewater spewed into the capital's harbour.

A wastewater pipe collapsed under Willis and Dixon Sts on December 20, necessitating the diversion of up to 100 litres of waste a second into the harbour.

The broken pipe serves the central city, taking waste into the main sewer that leads to Moa Point for treatment.

It's estimated five million litres of wastewater flowed into the harbour before an above-0ground bypass pipe could be put in place.

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Wellington City Council owns the infrastructure, Wellington Water is in charge of maintenance and managing the water services, and Greater Wellington Regional Council acts as the regulator.

It's immediately clear ageing infrastructure was the main cause of the collapse but it's GWRC's job to probe further into the causes and effects of the discharge.

Its investigation is still in early stages and is likely to take some time, a council spokesperson confirmed. Wellington Water is co-operating.

GWRC staff have been monitoring the waterfront and have undertaken water quality and visual assessments.

They noted Wellington Water's efforts to minimise the effects of the discharge.

Sucker trucks, which are like big tankers that slurp up sewage and then cart it off to a processing plant, were used at the site of the collapsed pipe.

The harbour has also been monitored to remove any "undesirable materials".

Potentially anything that gets flushed down the toilet could have been floating around.

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As recently as September last year Wellington Water was convicted and fined $67,500 for an illegal discharge of sewage sludge from the Porirua City Wastewater Treatment Plant.

A GWRC investigation showed that human error was the principal cause of that discharge. There had been a series of poor operational decisions and a failure to follow procedures.

Wellington Water issued an apology for the incident at the time and subsequently pleaded guilty.

During sentencing a Wellington District Court judge commented that, "there was a high degree of carelessness in this case … characterised by a cascade of mistakes".

"The public expects us to take non-compliance seriously," GWRC Environmental Regulation Team Leader James Snowdon said after the case concluded.

"Local authorities should operate their services adequately and safely. Whilst other options will always be considered, there comes a point when prosecution is required to hold individuals and organisations to account and to deter others from undertaking activities that may break the law."