Accident Compensation Corporation (ACC) is refusing to release information detailing the risks of its joint venture with Serco to build and run Wiri prison.
ACC invested in a partnership, Secure Future Wiri], in 2011 with Serco Group and two others to build the Auckland South Corrections Facility, known as Wiri Prison.
This was despite ACC, not being allowed to buy shares in Serco because of its links to nuclear weapons. Serco had been on ACC's exclusion list since 2008.
Documents released under the Official Information Act showed ACC's investment committee decided a partnership with Serco complied with its ethical investment policy.
But the government agency, which runs a $33 billion investment fund to help pay for the medical treatment it provides, withheld four pages of the report that detailed what it perceived to be investment risks.
Both the Labour Party and Green Party said the public had a right to know what those risks were.
"If there are risks to the investment and the investment's being done with public money then ACC should be prepared to make to make that information public, it is after all the public's money that's being invested," Labour's ACC spokesperson Sue Moroney said.
Green Party co-leader James Shaw agreed and said the amount invested by ACC should also be made public.
"Because ACC is a public fund designed to look after all our health and welfare, the public does have a right to know how much is co-invested with Serco," he said.
The documents showed ACC managers were aware that doing business with Serco could be seen as more "intimate" than owning shares in the company, but it disagreed with this view.
"Holding an equity investment implies a level of ownership. Therefore the equity-holder becomes somewhat responsible for the actions of the company. The important issue is ownership and control, not intimacy," it said in the report.
The 2011 report also noted that the Department of Corrections appointed Serco to run the remand prison at Mt Eden a year earlier.
"While the actions of government departments are not determinative for ACC's ethical investment policy, it does suggest that there is little risk of prejudicing New Zealand's reputation as a responsible member of the world community," it added.
Serco was stripped of its contract to run Mt Eden in July last year following allegations of assaults and organised fight clubs.
It was no longer a shareholder in Secure Future Wiri, but it still held the contract to run the prison until 2040.