The Comptroller and Auditor General says NAMA should have acted when it became known one of its advisors was getting 'fixers fees' relating to its Northern Irish loanbook.
Seamus McCarthy has been quizzed by the Public Accounts Committee (PAC) after his report suggesting the taxpayer lost out by up to €230 million on the sale of Project Eagle.
He says he has not had sufficient assurance that a different strategy from NAMA in the sale of Project Eagle would not have resulted in greater value for the taxpayer.
Mr McCarthy says NAMA should have done more about the involvement of Frank Cushnahan with some of the debtors.
"They could have written to Mr Cushnahan to ask him what was your involvement in relation to the Pimco bid," he explained.
"They didn't do it. They actually considered whether they should do that but they decided not to."
Mr McCarthy also said that "my view is that the allegations of Mr Cushnahan's in a fee-sharing arrangement warranted more action by NAMA when the issue came to light."
The PAC will hear from NAMA Chairman Frank Daly and its Chief Executive Brendan McDonagh this afternoon.
Michael Noonan has previously confirmed he will appear before the PAC to discuss the sale of Project Eagle.
Opposition TDs and the Taoiseach have agreed in principle to a statutory inquiry into the sale.