Nama has claimed an orchestrated campaign is being waged to intimidate and undermine the toxic assets agency.
Refuting allegations that the bad bank was leaking sensitive information and undervaluing loans, chairman Frank Daly said a lot of work had gone into the attack.
"I just feel it is designed to damage Nama, to undermine its credibility with the taxpayers of this country," he said.
Before a parliamentary watchdog, Mr Daly criticised what he called the "unquestioning credibility" given to the allegations and said the agency would not be cowed.
"I want it to be clearly understood that if this is a campaign designed to undermine us, Nama will not be intimidated, influenced or distracted by the efforts of whoever may be behind it," he said.
"In this agency, we have learned not to believe everything all of our debtors tell us - we verify what is presented to us and it would be very foolish not to do so.
"This week's events suggest that others unfortunately do not take the same basic precautions."
Nama said the allegations are coming from a former employee, Enda Farrell, who is being investigated by the Garda Bureau of Fraud Investigation.
However, Mr Daly said he did not know if Mr Farrell was being used by someone else in attempt to discredit the agency.
Nama chief executive Brendan McDonagh said it had carried out a review of all email correspondence, based on claims that a 'full file' on businessman Paddy McKillen was passed to a third party.
"There is no evidence from the search to date that any information relating to Mr McKillen was transmitted electronically," he said.
"With regard to this so-called 'full file', I wish to advise that Nama has very little information relating to Mr McKillen."
This was because the agency took a decision in July 2011 not to acquire Mr McKillen's loans, he said.
On claims that Nama was undervaluing loans, Mr McDonagh said the agency did not set property valuations.
This is done by professional valuers under international guidelines on behalf of banks and is then reviewed by a second firm of independent valuers.
In a small number of cases - around 12% - where the valuations don't match, a third valuer makes a determination, he told the Dail's Public Accounts Committee.
Mr McDonogh said Nama utterly refuted the very serious allegations.
He said: "It is clear at this stage that there has been a carefully orchestrated operation targeted at a small number of media outlets and Oireachtas members and its intended purpose is equally clear - to damage Nama and thereby undermine the financial interests of the state."