The cost to repair Christchurch's damaged well heads has surged by $21 million.
The Christchurch City Council said three months ago it would cost about 630 thousand dollars to protect some the city's 156 wells from contamination during flooding.
But a new report shows extra work costing about $21.5m is needed.
The council is not sure if this will extend how long the reticulated water supply will have to be chlorinated.
A council committee meeting will discuss the report today.