The ADB contributed $US5 million to the Nuku'alofa improvement project.
An ADB report says there's doubt the Government recognised the importance of maintenance of the improved assets.
Matangi Tonga reports continual maintenance of the drainage system is considered essential by the design engineers so it could do its job during periods of heavy rainfall.
The report reveals that cancellation of plans and reallocation of funds had compromised the goals of the five-year project.