An Audit Office report prompted by a whistleblower has found the cost of a government project valued at £46m has spiralled to almost £100m.
The LandWeb project was awarded to BT to provide online access to Land and Property Services (LPS).
The report found BT delivered a fully functional IT service, but criticised Department of Finance civil servants for "poor strategic planning".
In a statement, the department said they fully accept the report.
The Comptroller and Auditor General Kieran Donnelly said he was "alarmed" that mechanisms had not been put in place to secure better value for public money.
The project was due to run for 17 years up until 2016, but there have been a series of extensions to the contract, which is set to cost £107m by next year, 138% more than the original agreement value.
The Audit Office said individual users of the service were still facing excessive charges, labelling it "a form of taxation" which needed to be urgently addressed.
It was contacted by a whistleblower who raised concerns about "£100m being spent on the project, being wasteful of public money and lining the pockets of an IT contractor".
Stormont's Public Accounts Committee also raised concerns about the project 10 years ago.
Mr Donnelly said: "My report recognises that a fully functional and consistent IT service has been provided by BT throughout the LandWeb project to date.
"However, I am alarmed that mechanisms were not put in place to secure better value for money for citizens.
"Measures such as benchmarking, market testing and open book accounting should have been part of the Department's / LPS's agreement with BT, to ensure greater transparency and competitiveness.
"While the department did negotiate cost savings of £1.8m from the extension term 2019 to 2021, I found no evidence to clearly demonstrate that the LandWeb project has delivered value for money," he continued.
"This is particularly concerning in light of recommendations made by the Public Accounts Committee in 2010."
In their statement, the Department of Finance said it "is implementing its recommendation" and "as the report acknowledges cost savings of £1.8m have already been negotiated as part of a contract extension".
"The report also acknowledges that the department was unable to introduce a new fees order to manage surplus fees due to the absence of the assembly," the statement continued.
"The department is urgently progressing a revised fee order to be in place by 2021 and a procurement process in under way to replace the LandWeb system."