There are urgent calls for Canterbury's waterways to be tested, following an international study which links polluted water to bowel cancer

Canterbury medical officer of health Dr Alistair Humphrey is calling for urgent research into the levels of nitrates in the region's drinking water.

Humphrey told Radio New Zealand he became particularly concerned after reading the results of a huge study - which found a correlation between high nitrate levels and colorectal cancers.

The Denmark study found nitrate levels above 3.87 milligrams a litre substantially increases the risk of colorectal cancer.

Testing of 114 sites across the Canterbury Plains by Fish and Game found more than half of them were above this level.

Humphrey says the cause of the increase is intensified by dairy farming in the region.