The Wanganui District Council is trying to smoke out problems in part of its underground services.
The so-called "source detection investigations" are checking for possible cross-connections between the city's stormwater system and wastewater system.
Mark Hughes, council's infrastructure manager, said any cross-connecting of the two separate pipes meant stormwater was able to flow into the wastewater system during periods of wet weather.
Mr Hughes said the smoke testing is followed up by dye testing and finally a CCTV inspection.
The tests started in Whanganui East, in the Nixon St and surrounding area, and so far contractors had found nine possible cross-connections.
"In Whanganui East, we've identified nine possible cross-connections in different locations and we are currently undertaking dye testing and CCTV to confirm this. Once confirmed, we'll expose the pipes and remove the cross-connections," he said.
"We need to identify these cross-connections and isolate them because it will assist in reducing the frequency and volume of spilling from the wastewater system to the environment during wet weather events," he said.
He said some of these cross-connections happened during the initial stormwater-wastewater separation. The focus of the testing had now shifted to the central city with contractors working in the Harrison St area.
Mr Hughes said smoke testing will continue for another six weeks in the central city and then progressively move through other areas of the city.
Wellington firm Cardno has the contract for the work but it is only testing the public system and not private properties' connections.
This week, the contractors were pumping smoke into sewer pipes at the intersection of Niblett St and Plymouth St. Smoke soon came out of the stormwater sump indicating a possible cross-connection between the two systems.