Horticulture New Zealand says a water allocation plan in Waikato is putting hundreds of millions of dollars worth of produce grown in the region at risk.
The Waikato Regional Council's Variation 6 is being appealed by 37 groups in the Environment Court.
The controversial water regulations were first proposed in 2006 to manage the allocation and use of fresh water in the Waikato region.
Chris Keenan of Hort NZ says the variation places the region's annual $350 million fruit and vegetable production at stake because it does not ensure security of supply for those growers.
Other appellants to the Environment Court include Federated Farmers, Fonterra, Whenuanui Farms and Wairakei Pastoral Limited.
The hearing is in its fifth week.