The Hurunui River is at risk of "significant nitrate pollution" if Ngai Tahu converts a large chunk of North Canterbury land to irrigated dairy farming, opponents warn.

Ngai Tahu Forest Estates Limited (NTFE) has applied for a consent to change the use of land that may result in the discharge of nitrogen or phosphorus into the water.

In 2012, NTFE was granted consent by Environment Canterbury to enable the conversion of its Balmoral property, near Culverden, into a mix of dairy, dairy support, cropping, sheep and beef farming over a period of about 25 years.

NFTE proposes to convert 7000ha to irrigated dairy farming, another 617ha for dryland dairy farming and the remainder of about 979ha will not be grazed.

Green Party MP Eugenie Sage said the consent could create "significant nitrate pollution" of the Hurunui River and aquifers given the light, stony soils.

She said ECan must examine the application and the water pollution risks very carefully.

"The Parliamentary Commissioner for the Environment's recent report based on extensive modelling has shown that even with best practices in farm management, our rivers will become more polluted because of the scale of land use change and the increase in cow numbers," she said.

"Dairy conversion on the scale proposed risks condemning the Hurunui to a polluted, degraded future."

A Fish & Game spokesman said the land use change would impact negatively on the Hurunui River.

He said the combination of large gravels and shallow free draining soils would inevitably see runoff impacts that were higher in nitrogen and phosphorous.

Ngai Tahu Property chief executive Tony Sewell said NTFE was committed to best nutrient management practises, and there were no obvious adverse effects that would be more than minor as a result of the proposed activity.

He said Ngai Tahu was not an "environmental vandal", and was dedicated to looking after the environment.

Submissions close on March 21.