The Government has agreed to give the Board of Inquiry into the Ruataniwha Dam in Hawke's Bay a one-month extension to report back by 28 May this year.
The board wrote to the Ministers of Conservation and Environment, saying the issues are highly complex and involve significant conflicting expert evidence.
It says the Ruataniwha scheme is likely to be used as a prototype for freshwater management and land intensification in New Zealand so, given this importance, it needs more time to consider evidence.
It says there is significant conflicting technical evidence, particularly on water quality and allocation limits.
If approved, the $600 million water storage project would be the largest irrigation scheme in New Zealand and the largest dam to be consented in the 22-year history of the Resource Management Act.