Disaster recovery in rain-battered Zhengzhou in central China moved up a gear on Friday as floodwaters that deluged the city of 12 million began to recede, helping rescuers reach the trapped and stranded.

In neighbourhoods still immersed in waist-high water, rescuers used digger trucks, inflatable boats and other makeshift rafts to pull some residents to safety and deliver provisions to others trapped in high-rise apartment blocks.

Zhengzhou, the capital city of populous Henan province, has borne the brunt of extreme wet weather in central China this week, receiving the equivalent of a year's worth of rain in just a few days.

The resulting severe flooding killed 12 people who were trapped in the city's subway system. It also downed power supplies and stranded residents at home, in offices and on public transportation.

Some of the rescuers are volunteers using makeshift water craft, like the digger trucks deployed by local construction companies.

One of the volunteers, Li Kui, 34, said the demand for basic goods and foods was immense.

"We start our day at 8 a.m. and go on until 2 a.m. Besides having lunch and using the bathroom, we just go up and down the streets all day," Li said.

Asked if he was exhausted, Li said: "Yes, but compared to the people trapped inside, they must be feeling worse."

In other areas of the city where the floodwaters had subsided, municipal workers started the clean-up, sweeping away tree branches and clearing up other debris like marooned bicycles and scooters.

Tens of thousands of rescue workers, including the military, have been deployed across Henan more broadly. The death toll for the province from the flooding currently stands at 33.

Rescue professionals from neighbouring provinces have been called in, along with specialised vehicles to drain waterlogged streets, intersections and underground road tunnels.

While the rains in Zhengzhou had eased to a light drizzle, other parts of Henan were still forecast to receive heavy rain on Friday, according to weather reports.

In Xinxiang, a city north of Zhengzhou, 29 of 30 reservoirs were overflowing, a situation the local water conservancy bureau described as "grim".