Environmentalists say it's no surprise that the body responsible for providing Sydney with clean drinking water wants to exempt its catchment from new mining plans.

In a submission on the NSW government’s proposed changes to its mining planning policy, the Sydney Catchment Authority (SCA) has urged the planning department to exempt its catchment and other areas from the proposed amendments.

The SCA, which provides water to more than four million people in the greater Sydney region, says the amendments could pose a risk to infrastructure and water supply.

It noted that longwall mining has already caused significant damage to its infrastructure and lands, including watercourses and swamps.

Nature Conservation Council of NSW campaigns director Kate Smolski says the proposed planning policy mandates that mining and gas proposals be assessed primarily on economic benefits, rather than environmental or social impacts.

This was a concern when mining in the region had already seen the drying of river beds and the cracking of swamps, with four current projects draining three billion litres from the water supply each year, she said.

Ms Smolski said in a recent incident, longwall mining in the Lower Hunter caused large cracks to form.

When the mining company attempted to fill these cracks with concrete, the grout spilled into nearby creeks.

“They (the SCA) are really starting to see the impacts and that’s why they’re raising the alarm,” she told AAP on Tuesday.

Premier Barry O’Farrell says the new planning police would still offer protection.

“(The planning department) are not going to recommend for approval any mining project that will adversely affect Sydney drinking water’s supply,” he told Macquarie Radio on Wednesday.