The cost of setting up the Eircode postal address system was €20 million more than was initially budgeted for in 2009, the Dáil Public Accounts Committee has heard.
The final figure stands at €38 million, significantly more than €18m first estimate for the project, Comptroller and Auditor General Seamus McCarthy told the Committee.
The costs of consultants and Department staff were also not included in the early estimates for the project, the PAC members heard.
Furthermore, the estimates were based on a different type of address system than the one that was ultimately introduced.
Mr McCarthy said the financial benefits of the new post codes were also over-estimated originally.
He also said Revenue had already improved its data on addresses, following the Property Tax, before the system was introduced in July.
However, the Secretary General of the Department of Communications said many public bodies were using Eircode and more would do so in the months ahead.
Mark Griffin told the Committee that the codes had been used almost 3.7 million times since July and there are 20,000 "look ups" every day.
Meanwhile, the Health Service Executive says it expects to be using Eircodes for the National Ambulance Service by the end of February.
The HSE says it is testing their use within its system at the moment and reports that so far they are working.