The United Nations System in Papua New Guinea says it will be up to four months before commmunities can restore their food supplies following months of drought.
While there are now large areas of the country experiencing flooding, with a number of reports of deaths, drought remains in other places.
The UNDP country director for PNG Roy Trivedy said it was estimated many thousands of people remain in stress for lack of adequate food and clean water.
He said the flooding had been expected in the wake of the drought, but preparing communities for it was very hard.
"Communities don't necessarily have all the information," he said.
"The people like to think they are relatively safe and then we get places like in Jiwaka, the Waghi River flooding, lots of places across the country where there have been some landslides, some large landslides impacting on large numbers of people."