More than one million people in and around the Philippine capital battled deadly floods Wednesday amid relentless monsoon rains, with neck-deep waters trapping slum dwellers and the wealthy on roofs.

Sixty percent of Manila remained under water and vast tracts of surrounding farmland were also submerged as the deluge stretched into its third day, according to the government.

"The roads in some areas are like rivers. People have to use boats to move around. All the roads and alleys are flooded," civil defence chief Benito Ramos told AFP after surveying the megacity of 15 million people from the air.

The death toll from this week's rain in Manila and nearby provinces rose to 20 on Wednesday after four more people drowned, according to authorities.

This brought the confirmed number of people killed across the country since a typhoon triggered heavy rains in late July to 73.

State weather forecasters said more than 70 centimetres (27 inches) of rain -- well over the average for all of August -- had fallen in 48 hours, and warned of more to come overnight Wednesday.

The worst hit parts of Manila were mostly the poorest districts, where millions of slum dwellers have built homes along riverbanks, the swampy surrounds of a huge lake, canals and other areas susceptible to flooding.

In Santo Domingo, a creekside shantytown, mother-of-three Anita Alterano recounted how her family escaped the floods that submerged their one-storey home by walking over the roofs of houses until they reached high ground.

"We initially just decided to climb up on the roof where we were safe but wet. We waited for rescuers but it took so long for anyone to notice us," said Alterano, 43.

"So we got a rope, I tied myself to my husband and my children, we clambered from roof-to-roof... until we reached a school. But the problem is we have no water and food."

Some of Manila's richest districts were also affected, including the riverside community of Provident where water had inundated the ground floors of three-storey mansions.

Inside the gated village of about 2,000 homes, rescue workers on a motorised rubber boat drove past submerged luxury cars to retrieve children and the elderly from rooftops.

Across Manila and surrounding areas, 1.23 million people were affected by the floods, 850,000 of whom had to flee their submerged homes, according to the National Disaster Risk Reduction and Management Council.

Nearly 250,000 of them were sheltering in schools, gymnasiums and other buildings that have been turned into evacuation centres, while others were staying with relatives and friends, the council said.

A government worker told President Benigno Aquino in a televised disaster management meeting that the biggest problem for the relief effort was getting enough volunteers to deliver food, water and other emergency supplies.

Despite the chaos, the government ordered its employees and private sector workers back to their jobs on Wednesday, after closing down on Tuesday, while the stock market resumed trading.

The Philippines endures about 20 major storms or typhoons each rainy season, many of which are deadly.

But this week's floods in Manila were the worst in the capital since 2009, when Tropical Storm Ketsana killed more than 460 people.

The typhoons and storms typically start in the warm waters of the Pacific Ocean, then roar west towards the Philippines and onwards to other parts of southeast Asia, or further north to Taiwan, mainland China and Japan.

In China, authorities moved more than 1.5 million people out of the path of Typhoon Haikui before it slammed into the east coast on Wednesday morning.

China's financial centre Shanghai avoided a direct hit, but flights and some train services were suspended there and officials warned the biggest impact might be from rainfall later on Wednesday.

Haikui was the third typhoon to hit China in a week, with 23 people dying in the barrage of storms, according to Chinese state media.