People in Nigeria's flood-hit northeastern Borno state are struggling to get medical care as overwhelmed aid agencies warn of an outbreak of waterborne disease following the worst floods to hit the region in three decades.

More than 30 people have been killed by the floods, which authorities say affected about one million people, most of whom are housed in camps without food and clean water.

The deluge threatens not only the health and safety of the displaced but puts a strain on aid agencies and government resources, exacerbating an already critical humanitarian crisis.

The floods in Borno, the birthplace of Boko Haram militants in the Lake Chad basin, started when a dam burst its walls following heavy rainfall that has also caused floods in Cameroon, Chad, Mali and Niger, all part of Africa's Sahel region that usually receives little rain.

In the last two weeks of August, more than 1.5 million people were displaced across 12 countries in West and Central Africa due to floods, and about 465 have been killed, according to the United Nations humanitarian affairs office.

Over the weekend, an additional 50,000 people were displaced in northeastern Nigeria as the floods intensified, the Norwegian Refugee Council (NRC) said on Monday.

"The situation in the Sahel and Lake Chad region is increasingly dire, as the compounding effects of conflict, displacement and climate change take a severe toll on vulnerable populations," said Hassane Hamadou, NRC's Central and West Africa regional director.

The floods in West Africa come at a time of flooding in Europe after days of torrential rain that caused rivers to burst their banks in several parts of the region.

In a camp in Maiduguri, Borno's state capital, Bintu Amadu was among hundreds of frustrated people waiting for hours to see a doctor because her son had diarrhoea.

"We have not received any aid, and our attempts to see a doctor have been unsuccessful. We have been waiting for medical attention since yesterday, but to no avail," she said.

Ramatu Yajubu was happy she had obtained an appointment card after waiting for days, but quickly added: "I am uncertain about receiving attention due to the overwhelming number of people seeking care."

Mathias Goemaere, a field coordinator for Medecins Sans Frontieres, said that even before the floods, residents in Borno were struggling with malnutrition, following years of an Islamist insurgency that has driven people from their farms.

"They are exposed to their environment, so what do we see? A lot of waterborne diseases, diarrhoea, diarrhoeal diseases ... Malaria is around with a lot of mosquitoes," Goemaere told Reuters.

"So a lot of people, because of malnutrition, are immuno-suppressed, which makes them more susceptible to diseases."

Nigeria's government has separately warned of rising water levels in the country's largest rivers, the Benue and Niger, which could cause floods in the oil-producing Niger Delta region in the south.