Bad weather may have triggered a burst pipe underneath State Highway 1 in Wellington, but the real reason it failed is a problem that has been festering away out of sight.
The 4.5km wastewater pipeline runs between Paremata to a pump station in Porirua City.
A section of it burst after heavy rain that wreaked havoc across the country two weeks ago.
Wastewater spilled into the harbour before it could be temporarily repaired, then the pipe burst again less than a week later in a second dumping of rain.
A permanent solution will likely involve an entirely new pipeline beside the existing one.
During dry weather, around five million litres of wastewater flows through the pipeline daily.
Pipe fails well before its use-by-date
Two weeks ago the region experienced one-in-10-year rainfall.
Motorways were shut, businesses were flooded, trees came down, and people spent the night wondering if an earthquake was shaking their house or the wind.
The roughly 40-year-old wastewater pipe was put under pressure from larger than usual flow.
The pipe failed about 500m north of the Aotea Interchange.
The pipeline changes from an asbestos cement pressure pipe to a reinforced concrete gravity pipe around its midway point.
An NZ Herald analysis of Wellington Water data last year showed Porirua City has the highest proportion of asbestos pipes in its three waters network compared to the rest of the region's territorial authorities.
A third of the city's stormwater, wastewater, and water pipes are made from the material.
Only 13 per cent of Wellington City's pipe network is made from asbestos cement.
Unfortunately, the lifespan of these pipes wasn't properly understood when they were installed in the 1950s.
Asbestos cement was popular because it was cheap and could be manufactured in New Zealand.
But their popularity has now become a headache because they are failing earlier than expected.
Generally, well-installed pipes should last as long as 100 years but the average life of asbestos cement pipes is closer to 50 years.
Manufacturers stopped producing pipes made from asbestos cement in the 1980s.
The pipe between Porirua City Centre and Paremata was installed in the late 1970s.
It was expected to last at least 60 years, but at just 40 years old it has turned out to be in quite a state.
Patching up the burst pipe
Experts have been describing the pipe as being in a "fragile condition" even after the temporary repair - because part of it has been severely corroded by hydrogen sulphide.
The gas that wafts off sewage becomes sulphuric acid and then eats through the concrete, and eventually into the soil, if left unattended.
The top of the pipe, or the crown, was found to be fractured just under the median strip between the northbound and southbound lanes on State Highway 1.
To fix the broken part of the pipe, experts had to find a stable enough section to clamp in the repair.
Concrete-lined steel was put into 13 metres of the pipeline. Specialised fittings were brought from Auckland to make the joints.
Wellington Water chief wastewater adviser Steve Hutchison said safety and wellbeing of staff is a priority and crews on the ground have undertaken standard industry precautions for dealing with asbestos cement.
"They are trained to use specialised safe handling equipment and personal protection equipment."
The pipe could fail again in a different part so Wellington Water is managing pressure and flows to ensure it will last until a permanent solution can be put in place.
Wastewater ends up in the harbour
Up to 15 sucker trucks, also known as turd taxis, have been on call to prevent any further wastewater spillage into Porirua Harbour.
An unused pipeline across the harbour was also reinstated to alleviate pressure on the pipe the first time it was patched up.
But that had a fault in it and caused some wastewater to spill into the northwest area of Onepoto, opposite Papakowhai.
It was closed off when the overflow was discovered and Wellington Water has been repairing it so it can be used as a contingency.
Wellington Water customer planning engineer Sean de Roo said it was not possible to calculate the exact volume of wastewater that has gone into the harbour since the pipe broke because heavy rain diluted the wastewater.
A permanent fix
For now, operations have remained stable since the pipe was repaired for the second time on Sunday.
Hutchison said Wellington Water is continuing to investigate the extent of the pipeline's fragile condition, while exploring options for a permanent replacement or repair.
A permanent fix for the repaired pipe will likely be a second new pipeline in parallel with it, but the alignment would not necessarily have to be exactly the same.
The design, procurement, and installation is being fast-tracked. There's no firm time frame for the fix yet.
The Department of Internal Affairs recently proactively released expert evidence behind the Government's three waters reform proposal showing up to $185 billion is needed for pipes across the country over the next 30 years.
Porirua's share of that is estimated to be as much as $1.9 billion.
Wellington Water Committee chairman Campbell Barry has warned those numbers could balloon even further as issues continue to be identified during more extensive assessments of water networks.
Those assessments have been driven by unexpected failures in the region lately, the pipe at Paremata being the latest.
One thing is for sure, its replacement will not be made out of asbestos cement.