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.