Health volunteers in Malaysia have been preparing for any surge in waterborne diseases following devastating floods that saw tens of thousands of people evacuated, with concerns in the country that days of constant rains could be on the way.

Floodwaters have begun receding in the worst-hit state of Kelantan, on Malaysia's northeastern coast, but government data showed more than 40,000 people remained at temporary shelters on Wednesday amid fears of a potential second wave of flooding this week.

Videos of children playing in muddy floodwaters have gone viral online in recent days, sparking concerns over the potential spread of waterborne illnesses.

Nearly 5,000 cases of infectious diseases — including stomach flu and respiratory and skin infections — have been detected at Kelantan relief centres so far, government news agency Bernama reported on Tuesday, citing state health authorities.

Shelters prepared

The Malaysian Medical Relief Society, or MERCY Malaysia, said the number of reported cases have been manageable but its volunteers remained on alert for any future spikes.

"We have sort of stockpiled our hygiene kits at our base camp.

These are to be distributed if the surge of the community moving into the (shelters) happens again," the group's head of global operations, Shah Fiesal Hussain, told Reuters at a flood relief shelter in Kelantan's Pasir Mas district, near Malaysia's border with Thailand.

The volunteers have also prepared activities for children at the shelters to prevent their exposure to flood-related health risks, he said.

The Meteorological Department on Wednesday said it expects a monsoon surge from Dec. 8 to 14, which could bring continuous rain to the east coast of Malaysia's peninsular and parts of Sabah and Sarawak states on Borneo island.