The Public Accounts Committee (PAC) has called for a new Cost-benefit Analysis to be carried out before the final National Broadband (NBP) contract is signed.
In its sixth periodic report the committee said this is necessary to ensure that the full costs, including implementation, are known.
The PAC believes too that the procurement process for the NBP may have deterred interested companies from tendering for the contract.
The committee also said it was absolutely unacceptable that nobody can give a reasonable, accurate estimate of the expected final costs for the completion and fitting out of the National Children's Hospital.
The PAC recommends that in future no contract is signed for a significant capital project without a fully detailed design supported by a proper cost estimate.
In a statement, the Department of Communications, Climate Action and Environment said the recommendations contained in the PAC report regarding the Cost-benefit analysis are moot.
The Cost-benefit analysis for the NBP was updated throughout the process including after receipt of final tender, the statement said.