According to Vigicrues, the state warning service for flood risks, the flood has reached seven metres, a level not seen in valley of the Aude since 1891.
A woman was swept away by the water in Villardonnel and four other people died in Villegailhenc, Alain Thirion said on French television network BFMTV.
"At least one person was carried away in their nsleep – as to the others, we are not quite completely sure what happened at this stage," he added.
In addition, a person was injured in the collapse of a house in Cuxac and brought to hospital, according to police.
The police said that as the situation is likely to change as the day goes on.
"All my condolences to the families of the victims of .the floods," tweeted the Minister of Ecological Transition, François de Rugy, before the death toll increased to six deaths. He also praised "the mobilization of the services of the State, the department and the communes.
In the space of 5 hours, between 160 and 180 mm of water fell on the agglomeration of Carcassonne, the police said.
North of Carcassonne, all roads are cut off, according to the authorities. In this area, schools have been closed. The prefecture and firefighters have asked residents to stay at home.
The municipalities of Villemoustaussou, Villegailhenc, Conques, all of which have flood levels greater than two meters as well as Villardonnel, Floure and Trèbes are the most affected cities, according to the prefect. The water rose 8m in five hours in Trèbes and the flood could reach 10 metres by the middle of the day.
Flooding of the Aude
According to Météo-France, the central valley of the Aude has experienced "a flood of great magnitude" because of "very consistent input received from the tributaries of the Black Mountain" which were to continue "until morning".
In addition to the Aude, six other southern departments were on orange alert Monday morning including: Aveyron, Haute-Garonne, Herault, Pyrenees-Orientales, the Tarn, already on alert since Sunday, and Tarn-and- Garonne.