The Department of Conservation (DOC) has launched an inquiry to find out who leaked a secret report into a controversial dam proposal.

Conservation Minister Nick Smith is under fire over the department's draft submission on how the Ruataniwha Dam project in Hawke's Bay might harm the Tukituki River.

Opposition parties have accused him of leaning on DOC.

The draft expressed concern that proposed water management changes were "risky" and "untested."

DOC's critical 32-page draft report was reduced to two paragraphs, with no reference to water quality just two days after Smith met DOC bosses.

A leaked email, sent on the evening of the July 29 meeting, says Smith was "concerned" and wanted to see DOC's submission.

Now the department has begun a hunt to find out who leaked the original report and the email to the media, a spokeswoman for Smith confirmed.

Smith told reporters this morning that DOC officials were to blame for the debacle and an internal review is underway. He will meet with senior DOC figures this afternoon.

Last week Smith initially told reporters that he did not know of the existence of the submission saying: "There was no mention to me of a draft submission."

However, minutes of the meeting emerged later showing he had been given a short briefing on the direction the report was taking. This included assessment of the "potential effect of the proposed nutrient limits and targets on the freshwater values in the Tukituki catchment".

Smith admitted this morning he had been made aware of the report at a weekly status meeting with DOC officials.

Thirty-four items had been on the agenda, and just two sentences on the content of the Tukituki report, he said.

"What I knew was that a submission was being prepared," he said. "I knew that they had some preliminary views and I asked for a copy of it. That is what a responsible minister would do."

He received a copy two days later.

"It came to quite a different view from that earlier draft ... the question I've got for my department is having asked for the submission, why was it so different to what the draft was."

He had been advised of differences of views within the department, he said.

And DOC scientists came to different conclusions from Niwa on nutrient modelling, which would have been expensive to contest, he said.

"There are some questions within DOC internally and they are obviously reviewing their handling of the Tukituki case as to whether there was a bit of a disconnect between the senior management and the other DOC staff that were working on the document."

"I think it is proper that we have a bit of a review of how the Tukituki submission was handled ... We are talking about a $600 million dam and I would have expected a heads-up earlier that there were some issues to resolve about what submission DOC might make."

DOC had not handled the matter well, he said.

"There are some internal issues within DOC as to how it was managed," he said.

A spokesman for DOC confirmed an internal review was under way and that staff were being talked to about the leak of the documents.

He refused to confirm of claims made by Labour leader David Cunliffe that a staff member has resigned over the draft report.

Cunliffe said there was "a lot of murk" around Smith's role and he has a "lot of explaining to do".

"Suddenly, after Nick Smith's meeting with his officials a 34-odd page document turned into two paragraphs.

"He's said that he had access to it but he didn't read it. I think he's dancing on the head of a pin and there's a lot more to come out about this story.''