Canterbury's regional council ECAN is opposing plans by eight companies to expand their gravel quarrying operations near Christchurch.
The proposal would allow them to dig 3m below the groundwater level, 4m lower than the current restrictions.
A council report showed there was an increased risk groundwater quality would deteriorate due to potential contaminants in the back fill used to replace the gravel.
It said contaminated fill materials posed a long-term threat to groundwater that could persist for decades after the sites ceased operating.
A final decision on permission for the expansions will be made by independent commissioners after submissions are heard in a fortnight.