Ms Simons said water allocation and protection had become an issue of increasing concern, both locally and internationally.
"In this case water is a commodity over which one section of the community seeks the right to not only take but to contaminate. The issues are numerous and very complex," she said.
"The council is, essentially, the applicant, the proponent of a dam, and at the same time is also the regulator with the statutory duties to monitor, control, and maintain water quality and quantity. This raises another layer of issues and tensions."
Ms Simons said the Tukituki River, which would be impacted by the scheme, is already under threat from high nutrient levels.
"The council takes the position that improvement in water quality in the Tukituki River is, at this time, unaffordable. In other words, the current farming practices should endure because clean dairying or clean agriculture is unaffordable," she said.
"There is no avoiding, however, that a dam and the subsequent use of the water for intensifying agriculture will cause changes in the flow regime and increased [nutrient levels]. There will be consequent degradation of water quality, increases in periphyton growth, and significant stresses on ecosystem health."
The hearing is due to conclude today after final submissions from the council's lawyer, Trevor Robinson.
The board will then begin preparing a draft decision.
