"It is not much of a competition when you end up with only one supplier actively bidding in a framework, despite nine organisations being interested at the start.
"BT has been very cagey about its costs. BT's bids for the cost of street cabinets are on average 12% higher than the actual costs of similar work carried out in Northern Ireland."
She said: "Opaque data and limited benchmarks for comparison mean the department has no idea if BT is being reasonable or adding in big mark ups.
"Private sector organisations need to be 100% transparent about their figures when spending public money.
"It is not acceptable to hide behind arguments about commercial sensitivity.
A DCMS spokesperson said: "We have noted the NAO report and welcome its confirmation that processes we have put in place to ensure value for money are strong and robust.
"As the NAO report makes clear, the project's funding model greatly reduced the cost and financial risk to the taxpayer."
A BT spokesman said: "There was strong competition when prices were set at the start of the process and that has ensured counties have benefited from the best possible terms.
"Deploying fibre broadband is an expensive long-term business and so it was no surprise that others dropped out as the going got tough."
