The Green Party is challenging John Key to reveal what Treasury advice the National Party received on its proposed asset sales programme.
The Office of the Ombudsmen is investigating complaints from the Green and Labour parties over the Treasury's decision to withold information on the proposed programme.
Chief Ombudsman Beverley Wakem upheld a Treasury decision not to release documents under the Official Information Act relating to the proposed asset sales.
The Ombudsman said in a report that if these communications were to be released, officials would feel inhibited in the future when discussing each other's work or positions.
The Greens say the information is being suppressed just days out from the election and have written to the Ombudsman asking the office to reconsider its decision.
Green Party co-leader Russel Norman believes the Treasury documents will reveal weakness in National's partial asset sales plan.
Dr Norman says a thorough analysis of the proposal is difficult due to the withholding of critical information that should be in the public domain.
"Surely the people of New Zealand have got a right to know what Treasury actually thinks about this privatisation agenda and the advice they're giving the government."
He said that to suppress information prepared by government agencies for National about the asset sale proposal was "outrageous" and "an act of anti-democracy."
Dr Norman says if the National Party is certain their proposal is good, they should have no hesitation in releasing the information themselves to the public.
Labour Party leader Phil Goff also says all the information available should be made public.
"In three days, New Zealanders make a decision. John Key should come clean, be upfront and honest about this, tell us what assets he's going to put on the block."
National Party leader John Key says his party is not hiding anything in Treasury reports.
Mr Key says he's been advised that the information is commercially sensitive
During campaigning on Wednesday, he said state-owned company Genesis was likely to be the first company to be partially sold if National is in a position to proceed.
On Radio New Zealand's Nine to Noon programme, Mr Key again ruled out putting any more state-owned companies up for sale in the next Parliamentary term.
The Office of the Ombudsmen is considering its final recommendation on whether the information should be released and expects to announce this by midday on Thursday.