The board of inquiry considering the Ruataniwha dam proposal in Hawke's Bay has requested a one-month time extension.
It says the issues are highly complex and involve significant conflicting expert evidence.
The $600 million water-storage project would be the largest irrigation scheme in New Zealand and the largest dam consented in the 22 years of the Resource Management Act.
The board says the Ruataniwha scheme is likely to be used as a prototype for freshwater management and land intensification in New Zealand, and given its importance, more time is needed to consider the evidence.
It has proposed an end date to its inquiry of 28 May, which will have to be approved by the Ministers of Conservation and the Environment.