Public submissions on the $232-million water management project in Hawke's Bay's Tukituki Catchment closed in July, and hearings are due to begin in November.
An 83-metre high dam would store 91 million cubic metres of water in the upper Makaroro River so it could be used for irrigation.
Documents obtained by Radio New Zealand show DOC's 32-page draft submission called the proposal a risky and untested approach to water management, which could have hugely detrimental effects on water quality and fish species, and "kill" the Tukituki and Waipawa Rivers.
However, the draft paper was not submitted, and DOC instead gave a very brief, neutral submission, with none of those concerns mentioned.
Labour, the Greens and United Future want to know more details of DOC's concerns.
Labour's environment spokeswoman Maryan Street is blaming the government's plans to pare back the Resource Management Act's environmental provisions.
"Irrigating farm land for more intensive dairying is being prioritised over careful stewardship of our natural resources," Ms Street says.
United Future leader Peter Dunne says all relevant information about the proposal should be in the public arena, to allay fears the government is only presenting one side of the story.
There was a great deal of concern amongst recreational and environmental groups in Hawke's Bay about this proposal and the way it is being advanced, he said.
"A decision reached on only some of the information, and not all of it, will lack credibility and will be in no-one's long term interests."
Green Party conservation spokeswoman Eugenie Sage believes the government suppressed DOC's advice.
"The board deciding the matter will be getting a one-sided view... It will hear an 18-page submission from the Ministry of Primary Industries on why the Ruataniwha scheme should proceed, but only two paragraphs from DOC.