Provincial authorities have made a preliminary damage assessment but are relying on reports radioed in from isolated villages.
The flooding forced the water supply in Oro's provincial capital, Popondetta, about 150 kilometres northeast of Port Moresby, to be shutdown.
Popondetta's main airport and wharf nearby were also closed.
Disaster officials fear the death toll could rise significantly as dozens of residents from coastal and mountain villages are still missing after being washed away in surging flood waters, The Australian newspaper said.
Authorities have declared a state of emergency in several provinces, airlifting police, rescue workers and food supplies to some of the worse-affected areas.
|Heavy rains washed away bridges and roads|
About 85 per cent of the six million people in Papua New Guinea rely on subsistence farming.
On Monday Michael Somare, the prime minister, convened an emergency cabinet meeting saying the flooding had affected about 145,000 people.
"Continuous heavy rains associated with category 1 cyclone Guba over the last couple of days have severely affected the entire province," he said in a statement.
The governments of Australia and New Zealand, and other international aid agencies have said they are ready to send help once the extent of the disaster is known.