Mowhanau water supply users are being asked to boil water for human consumption until further notice.

"Our regular sampling programme picked up that we had e-coli counts in the water in the Mowhanau (Kai Iwi) system," Wanganui District Council Deputy Infrastructure Manager David Boothway said.

"E-coli is an indicator that the water may be contaminated. Therefore, on Tuesday (May 25) we hand delivered an advisory notice to all residents in Mowhanau to boil water used for human consumption."

The council is undertaking a number of steps to deal with the problem:

Flushing the entire Mowhanau water system (will be completed today). Inspecting the storage tanks for any contaminants (will be completed today). Increasing the free available chlorine level to increase the disinfectant level (completed). Undertaking daily monitoring.

"At this stage we do not know what has caused the contamination," Mr Boothway said.

"When we are satisfied that the monitoring indicates no contaminants then we will contact all the residents again and lift the 'boil water' notice."