NAMA has "categorically rejected" the key conclusions reached by the Comptroller and Auditor General in its report on the Project Eagle sale.
The agency said the key finding of the report is "fundamentally unsound and unstable and cannot be left unchallenged".
NAMA Chairman Frank Daly said in a statement: "It is clear to us that, if NAMA had retained the Eagle portfolio, there would be no investor interest in buying it today - or in the foreseeable future - at anything close to the £1.322bn price that was actually achieved".
The agency said the C&AG report "incorrectly assumes NAMA should apply the same discount rate to this poor quality loan portfolio as it applied to much higher quality assets in Dublin and London.
"As a result the report overstates the estimated value of this portfolio."
NAMA claims its position on this "key issue" of the discount rate is supported by four specialist international firms including KPMG and Lazards.
Mr Daly said the agency remains of the "strong view that the sale of the Project Eagle portfolio for £1.322bn was the best achievable outcome".
He added that the C&AG report produces no evidence to show the State would have been better off working out the property portfolio over a period of years.
NAMA said the C&AG position is unsupported by the market and their report was carried out by staff with no market experience of loan sales.
Mr Daly said: "We believe the report would have been very different if the C&AG had used the experts it had acknowledged were necessary prior to carrying out this review.
"The C&AG tried to address this by beginning a process last year to bring in this external market expertise but it did not complete this process."
Frank Daly: Restrictions by NAMA on bidding process over NI portfolio did not reduce number of bidders https://t.co/pmSJMgVzb5— RTÉ News (@rtenews) September 14, 2016
Mr Daly said it was the agency's view that a discount rate in the 10% - 15% range would have been appropriate to apply to the cash flows associated with the Eagle portfolio.
He said the price achieved - £1.32bn - fell within the commercially acceptable range of values between £l.25bn and £l.36bn.
That range was generated by the discount rates that were applied by purchasers or investors bidding for the Eagle portfolio.
Mr Daly has written to the Minister for Finance outlining the agencies fundamental disagreement with many of the conclusions reached by the C&AG.
In the letter, which NAMA published on its website, Mr Daly says the difference in price between NAMA and the C&AG is a "technical valuation point" relating to discounts on assets based on the future cash flow arising from the loan portfolio.
Speaking on RTÉ's Six One, Mr Daly said the C&AG is of the view that NAMA should have put a different value on the loans, but its view was it was £1.3bn and that is what they did.
He said the agency does not accept the report's assertion that restrictions imposed by NAMA in the bidding process reduced the number of bidders, but admitted that some of them expressed dissatisfaction with the process.