The installation of water meters around the country cost about €100 million more than originally envisaged by Bord Gáis in estimates given to the Government.

Official correspondence obtained by RTÉ's This Week programme drawn up in May of last year showed the Department of the Environment believed the entire cost of the metering programme would come to €431.56 million.

In the documents, the Department of the Environment said its understanding of the overall budget was based on figures provided by Bord Gáis.

However, several weeks later it emerged that the final cost of the metering project would be €539 million – or about €107 million higher than the original estimate.

RTÉ reported that the detailed estimate given by Bord Gáis to the Department of the Environment had been carried out by external consultants working alongside the company's own staff.

In a statement Bord Gáis (now known as Ervia) said: “The earlier estimates were developed prior to detailed survey work designed to find out where and how many meters should be installed. The final budget emerged from this survey work and the development of detailed contract documents. With capital projects of this scale such changes between initial estimates and final contract costs are common, particularly when there is no comparable precedent.”

“The final budget figure reflects the outcome of the detailed scoping of the project and a very competitive procurement process whch took place under EU regulations. That procurement process has proven to be very successful, with the appointed contractors delivering the meter installation programme ahead of target so far.”

RTÉ said the document, in which the Department of the Environment had noted its understanding of the likely costs of the water metering project, was dated May 22nd, 2013.

The following month, at its first official board meeting, Irish Water awarded the main contracts to the successful bidders for the installation of meters nationwide, subject to further negotiations.

The detailed costings given to the Department of the Environment included a breakdown of 20 separate sub-components of the likely costs.

The costs included everything from the meters, boundary boxes, ancillary works and management costs to smaller items including miscellaneous media and print material.