Manawatu-Wanganui Regional Council says it has cleaned up as much of the diesel which polluted the water supply at Raetihi last year as it can.
About 19,000 litres of diesel fuel leaked into the Makotuku stream in Tongariro National Park from a tank at the base of a skifield run by Ruapehu Alpine Lifts.
Since then Raetihi has been using an alternate water source.
The regional council says there is still some contamination within the scree slopes on Mt Ruapehu that has been deemed to impractical to recover.
Chief executive Peter Till says there is a risk that some of that diesel could make its way into the Makotuku during heavy rain.
A hydrocarbon sensor has been installed which he says will shut the water supply if necessary and allow the council to switch to the Makara stream.