Some environmentalists liken the waste that would be created by the proposed expansion of New Zealand King Salmon's farms in the Marlborough Sounds to that made by people in a large city.
The salmon company wants to add nine more farms to the five it already has and is presenting its plan to the Environmental Protection Authority.
It says the proposed expansion would mean it would be farming a total of 14ha. The surface area of the Sounds is 150,000ha.
The application has attracted 1400 submissions, of which 800 oppose the development and 400 support it.
Counsel for environmental group Sustain our Sounds, Warwick Heal, says the effects on the water would be "catastrophic", like that made in a city of 400,000 people or 250,000 cows.
New Zealand King Salmon says this analogy is grand-standing and untrue.
Chief executive Grant Rosewarne says the analogy is false.
The other main opponent is Marlborough District Council.
King Salmon has spent $9 million on the application. It is seeking to create more than $200 million of additional value.
Mr Rosewarne says the company needs to expand to meet demand and it would have to consider expanding to Chile or the United States if the application is not successful.
The hearing continues on Tuesday.