Federated Farmers says a dam proposed for the Tasman district can only be affordable if it has government support.
Public consultation into the funding models for the Waimea Community Dam highlighted concerns over projected costs for the dam.
The Tasman mayor said the dam in the Lee Valley could cost up to $80 million, although some opponents have suggested a figure in excess of $100 million.
Richard Kempthorne said access to more water will help safeguard the district's financial security.
Federated Farmers Nelson provincial president Martin O'Connor said it is not a matter of whether the dam goes ahead but how it is funded.
He said the construction will cost irrigators around $500 per hectare and increase their rates by 11 cents for every per cubic metre of water used.
Mr O'Connor said central Government needs to back the scheme so ratepayers are not left to foot the bill.