ACC responded to the latest request, saying its position would be prejudiced if it disclosed whether the information asked for even existed.
"ACC neither confirms nor denies the existence or non-existence of the information," it said.
However, Ikaroa-Rawhiti MP Ms Whaitiri said, as a Crown organisation, ACC should be transparent with New Zealanders and state whether it is investing in the dam.
"Hawke's Bay ratepayers are fed up with the endless speculation over the 'mystery investor' in the dam," she said.
"Millions of dollars of public money has already been spent on Ruataniwha and Kiwis deserve to know if even more is about to be sunk into this white elephant," Ms Whaitiri said.
She said it was also concerning that ACC Minister Nikki Kaye was not aware of whether or not the fund was planning such a significant investment.
A spokesman from Ms Kaye's office said Ms Whaitiri needs to understand that, under the Crown Entities Act, the minister has no involvement in the decision making process for ACC investments.
"The responsibility for managing ACC investment funds resides with the ACC Board."
Meanwhile, the Green Party is calling on ACC not to invest public money into the dam, saying it could be a disaster for the Hawke's Bay environment.
"The case for the Ruataniwha dam doesn't stack up economically or environmentally, and ACC shouldn't pour New Zealanders' money into it when even private investors won't," Green Party water spokeswoman Catherine Delahunty said.
As it stands, the dam scheme has failed to attract any publicly known private investors, with Ms Delahunty saying it could end up relying on fully on public funding.
"ACC should be using its investment power to help clean up our rivers, not contribute to their decline by funding projects like the Ruataniwha dam."
