New information has come to light about the rare Maui's dolphin - but the government will keep it secret until after it has finalised a plan for the species' future.
Previous estimates have said only 55 of the adult dolphins - the critically endangered North Island sub-species of the Hector's dolphin - are left.
Maui's dolphins are occasionaly spotted in Raglan, and last year four were filmed near Manu Bay.
Scientists predict the sub-species will become functionally extinct in less than 20 years unless it is better protected.
The government has been working on a Threat Management Plan for the dolphin - which may include further limiting fishing restrictions in its habitat - but its publication has been delayed since January.
In August, Conservation Minister Nick Smith said at a conference it would be delayed again after "new information" came to light about the dolphin.
It said it was seeking advice on the significance of the new information, which related to the "population and extent of the presence of Maui dolphins", and how that related to the Threat Management Plan.
Information about the dolphin is highly sought-after. Sightings are rare, and population studies are time-consuming and costly.
Since Smith announced the delay, requests from media, opposition politicians and advocacy groups for access to the new information have all been turned down.
In an OIA response to Fairfax Media this week, Smith's office said it could not provide the data because it would "likely unreasonably prejudice the commercial position of the person who supplied, or is subject of, the information."
The public interest was not outweighed by other considerations, Smith said.
Green MP Gareth Hughes said the government needed to come clean about the new information and the continual delays.
"The Government should not hide behind a 'commercial interest' excuse to refuse to answer legitimate OIA requests," he said.
"With only 55 adult Maui's Dolphins left the Government needs to act with haste to protect the last dolphins from the threats of lethal fishing nets to give them a shot at survival."
Smith's office has not said when the final plan will be released.
Maui's are the world's rarest dolphins. They measure only 1m long and are characterised by their dorsal fin, which resembles a Mickey Mouse ear.
They live along the west coast of the North Island between Taranaki and Auckland.