Aid agency Oxfam says badly designed flood defences may have worsened the recent deadly South Asian flooding.
The UN fears much of the farming land has been lost
Oxfam said poorly built or maintained embankments on rivers obstructed the natural flow of water and caused more extensive flooding downstream.
It urged more to be done preparing people for floods to avoid post-disaster salvage spending.
Figures for the death toll vary widely from 500 to 2,300. At least 20 million people were affected.
Oxfam's report says the construction of embankments in the Indian state of Bihar, for example, had increased the flood prone area in the state from 2.5m hectares to nearly 7m.
And in Bangladesh, poor maintenance led to 75 embankments being breached by floodwaters during the recent crisis.
The report says new investments in dams and embankments should be avoided and that renewing local water supply systems and drainage works would be more effective.
It advocates long term planning to prepare communities for floods.
Measures that have been proven to work, the report says, include building flood platforms, raising houses on stilts and providing communities with basic first-aid training.
In part of the Indian state of Assam, it says, community-based disaster preparedness costs just 2% of the projected expense of post-flood relief.
Many people in flood-affected areas are returning home to count the cost of the floods, although tens of thousands are still living in shelters.
THE ASIAN MONSOON
Monsoon winds blow north-easterly for one half of the year, and from the south-west for the other half
South-westerly winds bring the heavy rains from June to Sept
Winds arrive in southern India six weeks before the north west
Annual rainfall varies considerably
Dukhia Manjhi, who lives in Bihar, told AFP news agency: "Nothing - there's nothing to go back to... The floods have washed away the fertile soil and there's nothing to rebuild our home."
Vinoy Ohdar, of aid group ActionAid, told AFP: "So many people have lost their homes and these are mostly the poorest of the poor. Re-establishing their livelihoods is going to be a major issue."
Heavy monsoon rains have struck many parts of north India, from Assam in the east to Gujarat in the west.
Tens of thousands of people who fled to relief centres in Bangladesh are also returning home.
But tens of thousands more are reported to have been hospitalised for diarrhoea and other water-borne diseases.
The UN has warned that north India and Bangladesh could suffer acute food shortages following the loss of farming land to floods.
Scores are reported to have died and hundreds of thousands displaced in southern Nepal.
The Associated Press also reported 22 dead in a landslide in Kohistan district in north-west Pakistan.