UPDATED 12.18pm: A public conference on the Ruataniwha Dam project has heard authorities may not be able to stop hundreds of farms leaching dangerous nitrogen into waterways.

The Ruataniwha Board of Inquiry is hearing submissions today on its consent conditions granted for the $275 million project.

Three environmental groups successfully challenged the board's decision to exempt 615 large farms from new stringent water quality rules.

Counsel for the Hawke's Bay Regional Council, Trevor Robinson says more than 70% of the farms have very limited scope to control nitrogen run-off.

He says the council would have to take a 'see how it goes' approach and put in more regulations, if nitrogen levels are deemed too excessive.