Improvements to Tonga's roads and drainage are "less than likely to be sustainable" if not maintained, warns the Asian Development Bank.
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.
The Sopu drainage channel was cancelled and funds were reallocated from core projects to non-core sub-projects in the Tonga Integrated Urban Development Sector Project, started in 2008.
The report notes that changes, overlaps with a Chinese-funded aid project and reorganisation of the government's Ministry of Works, created set-backs for the project.
There was also missing information which cast doubts over its future even though the project was completed on time and within the approved budget.
