The incident marked the beginning of Wellington's water woes.
The discharge was triggered after a wastewater tunnel collapsed under Dixon St in the CBD, which took months to repair.
Investigation documents obtained under the Local Government Official Information and Meetings Act conclude the discharge was allowed under emergency works provisions in the Resource Management Act (RMA).
The discharge was to prevent damage to property and health risks, specifically a "significant and uncontrolled" flow into the vicinity of Dixon and Willis Sts.
As well as what went into the harbour, the investigation found there may have also been a discharge of wastewater to the ground immediately adjacent to the collapsed tunnel.
That discharge was not allowed for under the RMA, but GWRC said it accepted it was due to mechanical failure that was beyond Wellington Water's control.
No enforcement action over the incident has therefore been taken.
Wellington Water has however been ordered to pay $7,728 for the costs associated with compliance monitoring.
Work to trench a new wastewater pipe under Willis St took months to complete. Photo / Georgina Campbell
GWRC noted Wellington Water took several measures to minimise the overflow into the harbour and mitigate the effects.
These included contractors working around the clock to construct an above-ground pipeline, the use of sucker trucks, health warning signs, security guards, and contact with local iwi to establish a rahui.
Wellington Water chief executive Colin Crampton said the incident was a critical asset failure and was managed appropriately under the circumstances.
He said the frequency and scale of regular condition checks were now being increased.
"In our day to day role we aim to carry out our work in accordance with the consents we have obtained.
"In circumstances where things don't go right, we always try to act with urgency and endeavour to find options which either do not affect the environment or minimise the impacts to the environment."
Wellington Water apologised to the community at the time of the incident.
Crampton said every effort was made to avoid wastewater flow into the harbour.
"We used storage under the Michael Fowler carpark and sucker trucks to transport wastewater to another part of the network where it could be pumped into the interceptor.
"We immediately commissioned an overload pumping system to bypass the blockage caused by the tunnel collapse."
Wellington Water's latest annual report has revealed sewage spills are 20 times that of the company's target.
In the past year there were 2096 spills, knows as dry weather overflows, compared with a target of less than 100.
The majority of these blockages were caused by tree roots, fats, wet wipes, and were aggravated by pipe failures such as cracks and dislocations.
Wellington Water said the target would have to be adjusted to reflect an ageing network.
Wellington Water Committee meets
The annual report was presented at a Wellington Water Committee meeting yesterday as well as an economic case into residential water metering across the region.
Chair David Bassett told his colleagues they only needed to look at the executive summary of the economic case to realise the challenges the region was facing.
"Current demand for water in the Wellington Metropolitan Region is set to exceed supply in the near future and a new water source could be required between 2026 and 2030", the summary said.
Data for the 2019/2020 year suggest water losses across the region of up 31 per cent. Photo / Getty Images
Water metering is being looked at as a way to reduce demand to defer a $250 million investment in a new water source.
It could do so by 20 per cent, not so much by influencing consumer behaviour as through identifying leaks in supply pipes and fittings.
Data for the 2019/2020 year suggests water losses across the region of up to 31 per cent.
"This means that potentially close to one third of water that we collect, treat and supply is lost before it reaches the customer", meeting documents said.
The average household water use in the Wellington Metropolitan Region is more than 200 litres per person per day.
That is significantly higher than other major cities such as Auckland, about 160 litres per person, and Melbourne, about 150 litres per person.
Porirua Mayor Anita Baker said the region needed metering sooner rather than later.
She said there also needed to be a proactive push to educate people about water conservation.
"In Auckland they don't brush their teeth with the tap running, in Wellington they do. Have a bucket in the shower to save the water for your garden.
"And all these damn watering systems that run the minute the water restrictions come off, at 7pm or 9pm, frustrate the living daylights out of me. I'd like to see people fined."
South Wairarapa already has meters and Mayor Alex Beijen urged his colleagues to install them too.
"Do it and do it now, it's a crucial part of being able to detect leaks. The sooner you do it, the sooner it becomes normalised."
But Upper Hutt Mayor Wayne Guppy said building more bulk water storage needed to be the priority after 12 years of procrastination on the subject.
