The collapse of a critical wastewater tunnel under Wellington's CBD was decades in the making after it was left unchecked to wither in the grasp of sulphuric acid.
More than six million litres of wastewater spewed into the harbour just before Christmas as a result.
A huge hole underneath Dixon St was discovered two weeks before the collapse, but it was too late.
It was so bad there were fears the road could cave in putting surrounding buildings at risk.
Nearby streets narrowly avoided being flooded with wastewater.
An independent review said the cost of dealing with the unexpected failure far exceeded the cost of planned inspections and upgrades.
The review, quietly published on Wellington Water's website on Thursday, has revealed the collapse was a totally avoidable emergency.
Wellington Water has since detailed the review's findings, after the Herald asked questions and published an article.
A report as far back as 2004 identified an area of significant corrosion in the place the tunnel later failed, and recommended further inspection on a 10-year cycle after repairs.
But that report was archived and lost as the management of Wellington's water twice changed hands between then and now.
Inspections between 2013 and 2018 were based on observations from 70 metres upstream because the part of the tunnel in question was costly to get to.
It probably would have been considered money well spent if Wellington Water staff had known about the 2004 report.
Instead, a tunnel already in poor condition, sat there for 16 years with sulphuric acid eating away at its concrete walls.
It wasn't until December 6 last year that a massive cavity was discovered by chance.
Contractors were looking into ways to improve the local network when they came across a gaping hole thought to be as close as 1m from the road surface.
The alarm rang out and an emergency response team was swiftly pulled together.
The cavity risked the road collapsing, which could have damaged to facilities and nearby St Johns Church and the Trinity Hotel.
Disturbing the surrounding area would risk further collapse, but the void could not be left untouched. Experts needed to come up with a solution and fast.
Wellington's weather did not play ball.
The day after the cavity was discovered more than 8mm of rain fell in one hour from 6pm.
The edges of the gaping hole continued to waste away and the tunnel started to become blocked.
Suction pumps were installed to pump wastewater downstream of the cavity and work was ramped up to clear an old wastewater trunk main down Willis St.
After a week of considering the best way to minimise ground movement and reduce further risk of collapse, a temporary structure was put in place.
Residents were notified via a letter drop, although those delivering them ran into difficulties gaining access to mail boxes in secured inner city apartment blocks.
Contractors were midway through installing the third pile to support the temporary structure when the cavity completely collapsed causing a major blockage.
It was a week before Christmas Day.
Sucker trucks were called in to try to remove the material, but success was limited and the tunnel had become completely blocked by December 20.
The call was made by 8.30am to switch off pump stations, diverting wastewater away from the collapse to prevent a "significant and uncontrolled" flow into the vicinity of Dixon and Willis Sts.
The cavity was stabilised by filling it with quarry material.
Wellington Water scoured places that could hold the wastewater that had nowhere to go, including pump station wet wells and the Michael Fowler Storage Tank.
This was supplemented by sucker trucks working around the clock to remove about 1.45 million litres of wastewater.
Nevertheless, over two days, 6 million litres of wastewater was discharged into the harbour.
More than 60 staff were dispatched to the waterfront to ask businesses to reduce water consumption to keep wastewater flows to an absolute minimum.
Signsand security personnel warned the public not to swim in potentially contaminated water. Crews then spent four days retrieving floating matter from the sea.
The day before Christmas temporary solutions were in place, most significantly an above- ground pipeline to move wastewater down Willis St.
The work to get the diversion in place was done with urgency and many staff members worked extended hours.
The independent review said stories of around-the-clock work shifts were not uncommon.
Work to trench a permanent new wastewater pipe under Willis St has been forecast to cost up to $4m.
The collapsed tunnel is a critical asset so should have had a routine inspection programme.
Instead, Wellington Water staff were barely aware of its existence let alone what condition it was in.
The independent review reported strong anecdotal evidence that more money for asset renewal programmes was regularly sought from Wellington City Council, and regularly declined.
The review said it appeared the council has tended to set budgets against which Wellington Water developed its work programmes to best achieve service targets.
But it's not as simple as Wellington Water asking for more money and not getting it.
The assertion is anecdotal - there is no documented evidence of such requests.
Wellington mayor Andy Foster told the Herald this week he was unaware of any approach to the council as an elected body asking for additional resource that was ever declined.
However, he said he has been told approaches were made to individual elected members.
The review also noted that Wellington Water needed to clearly articulate its recommendations and expenditure required.
"It also needs to clearly articulate the impacts and risks faced by departing from the recommended programmes.
"Although there is anecdotal evidence that discussions were held with [the] council, we were unable to find strong evidence that these impacts and risks were clearly articulated following budget amendments."