About 2000 tonnes of cement-like grout have blocked up a big new water main pipe in central Auckland.
It is having to be dug out after a big pour on the $400 million Hunua 4 water supply project went wrong.
It appears the equivalent of more than 150 mixer-trucks full of grout got poured before contractors realised it was going into the underground pipe instead of around it to support it, as planned.
Four months on, repair work is still going on near Khyber Pass.
"Our contractor is working to remove around 1000m3 of grout from the pipe," Watercare told RNZ today.
"Grout is not as strong as normal concrete but is still challenging to remove."
Watercare is still not sure what went wrong on the final section of the project.
"We need to finish removing the grout in order to identify how the incident occurred."
The repairs were being dealt with by its insurers and would not delay the final section much, it said.
Most of the 31km-long, 1.6m-diameter pipe - inside a 3m-diameter concrete casing - is already in use, having been completed in stages.
The final stage of the nine-year project runs from Epsom to Khyber Pass, and is being installed mostly by tunnelling to reduce disruption.
The pipe is laid in the tunnel, then the gap between the pipe and the tunnel is filled with grout.
"Thankfully, we are able to carry out other work while we remove the grout to minimise delays to the overall project schedule," Watercare said.
It reported the problem to Auckland Council, its owner.