Stormwater pipes in Wellington's Ōwhiro Bay will be flushed out today in an attempt to clear faecal matter from cross connections.
Cross connections are when sewage pipes have been incorrectly joined to the stormwater network, or vice versa.
The flush comes after a spike in faecal contamination in the Ōwhrio Stream and harbour outlet - measuring up to 160 times higher than the level considered safe to swim on 1 March.
The next day, it had dropped well within safe levels again.
Wellington Water spokesperson Alex van Paassen said these spikes can happen with rain, which can lead to higher flows in the stormwater pipes which can wash down faecal matter.
Van Paassen said over time, sewage gets stuck to the inside walls of the pipes. The flush acts "like a toilet brush" that cleans the inside of the pipes, and vacuum trucks at the other end collect the outflow, he said.
So far five cross connections have been found and fixed in the catchment.
Most cross-connections are from private pipes into public (council) pipes, and this is the case here, van Paassen said. More are believed to exist, so investigations are ongoing to determine the full extent of the problem.
Dye testing is being used to find others and this will continue over the next few weeks. Dye is put down the gully trap - the part of the pipe that receives the wastewater - of a property.
If the dye comes out in the stormwater network, there is a plumbing fault.
Community problems 'troubling to hear'
Wellington Water chief executive Colin Crampton visited the Ōwhiro Bay community last Saturday to get a sense of the issues at hand.
The visit followed an RNZ story last week that revealed the stench residents in Ōwhiro Bay were dealing with, which they said was coming from the sewage traetment plant at the Southern Landfill, and eminating from the 'turd taxis' travelling along Happy Valley Rd.
Ōwhiro Bay resident Eugene Doyle has been leading a community effort about the contamination and "tsunmai of faecal matter" that locals were dealing with.
Doyle said the community were vocal about the issues they were facing, including not being able to swim in the Ōwhiro harbour and the "foul, pestilential" sewage smell.
In a letter sent to Doyle after the visit, Crampton said he "accepted the community's views" that the trucks smell, and that there was a smell drifitng over the bay.
"It's pretty troubling for me to hear that the community has lost trust in the safety of the bay for swimming and in the overall safety of the stream. You are quite right when you say there is an opportunity to show up and work together to find a way forward," the letter stated.
"I am gratified that the community understands why we have to truck the sludge slurry because the alternative would be environmentally unacceptable. The question is when will it stop."
Part of the smell last week had come from a sewage overflow at the sludge treatment plant, which happened due to two mechanical failures.
Promise of transparency over treatment plant
In the letter, Crampton promised that Wellington Water would work with the sludge treatment plant contractor, Veolia, to make the running of this plant more transparent.
Regarding the contamination levels in the Ōwhiro Stream, Crampton said the baseline water quality had been damaged by previous generations that viewed the stream as an acceptable relief valve for our wastewater system.
"We all agree this is totally unacceptable and we need to begin the long journey back to swimmable, ecologically good and finally as pure as it can be."
He added that Wellington Water woud be pursuing the plumbers responsible for the five cross connections that had been fixed so far "because it was their primary accountability to do the job right".
"The last question is how do we stop this happening again with all the development going on in the Owhiro Valley."
Crampton said Wellington City Council funded Wellington Water to a level of service that only covered chasing pollution sources that closed beaches. "We are not funded to progressively improve the base level of pollution in the stream which is what you want."
Wellington Water has promoted the idea of "roving crews" to all six councils in the Greater Wellington region. These crews would work through a catchment, proactively fixing all faults so would ultimately improve the baseline.
So far, Porirua City Council is understood to be the only one that has adopted this idea.