While the Ombudsman chose not to have the information released, it revealed the Government's assurances were based largely on the views of ministerial employees and on informal discussions with finance industry figures.
One News reported that Treasury admitted it had not provided Minister of Finance Bill English with any advice about the workability of a 10 per cent shareholding cap.
"It looks like National has deceived New Zealand, tried to cover it up and been caught out," Labour leader Phil Goff said.
One News also reported a significant amount of information about the asset sales plan was being withheld from the public, including five official reports.
However, the Ombudsman upheld the Government's decision not to release the documents saying disclosing the information could affect the returns gained from the sales, in turn having a significant economic impact.
Labour campaign spokesman Grant Robertson said Mr Key and other senior ministers had been caught out "attempting to hoodwink the public".
Yesterday's report on the advice backing National's assurances over the asset sales programme "weakens an already flimsy case" for the plan, he said.