The "secret letter" was first referred to by the Senator for South Australia, Simon Birmingham, who asked the Government a question regarding the secrecy of the NBN expert panel report in Senate Estimates.
Patricia Scott, chair of the Expert Panel and secretary of the Department of Broadband, Communications and the Digital Economy (DBCDE), brought to light a letter, separate to the expert panel report, which gave the Government its recommendation to abandon the Request for Proposals (RFP) process.
"The panel came to the view that it was also in a position to tender some advice to the Government [in addition to the report]," Ms Scott said in Senate Estimates.
"Because it was a sort of second step, it was separate to the report."
Scott said the letter is also confidential, but refused to say whether it contained any analysis of the bids put forward by RFP tenders.
The Government has long refused to release documents relating to the RFP due to the "commercial confidentiality" it promised bidders.
Reginald Coutts, one of the members of the expert panel, has since told iTnews that he and his fellow panel members provided advice in addition to the expert panel report that prompted the government to abandon it.
"What he [Senator Conroy] had was a report from the panel, he had a report from the ACCC and he had correspondence [in additional to the report] from the panel about a way forward," Coutts said.
Senator Minchin today said that he wanted the Government to release the confidential letter.
"Considering the strong public interest, Senator Conroy must release the secret letter to show Australians exactly what the expert panel advised and when," he said.
"If the Government is simply following the advice outlined in the letter, why won't it release it to confirm that claim to Australian taxpayers?"
Senator Conroy has previously refused Minchin's requests for disclosure of confidential NBN documents.
When asked by Minchin in Senate Estimates for information on the report Conroy told him to go ask the panel members himself.
"You are welcome to ask Rod Tucker, Reg Coutts, John Wylie, Tony Mitchell or any of the members, 'Has Senator Conroy or the government sought to mislead you on the report?'," Conroy said in Senate Estimates.
"You are welcome to phone them and ask them. They are all publicly available and they have all been consistent with what we suggested."