Bangkok is used to absorbing the sudden afternoon downpours that inundate the city during the rainy season. Even streets in many central high-rise districts can briefly turn into rivers, with rats and cockroaches racing out of sewers and taking temporary refuge on trees and buildings as the subterranean world is flushed out.

But every city has a breaking point, and this year rain fell so fast and so hard that whole neighbourhoods of the Thai capital have been transformed, not into rivers, but stagnant lakes. In parts of the city, roughly 300mm of rain – the equivalent of half a year of average rainfall in London – fell in about two days.

About 40 people died in flood-related incidents, according to the government. A number of the deaths were from drowning and several from accidental electrocution. Thousands of people stayed in more than 230 shelters across the city, authorities said.

Suleemas Kurangkul, a 73-year-old resident of a block of flats whose ground floor stairs are still under water, said last weekend’s rain was not the usual evening storm but was constant, “like all day, all night”.

“The water just kept rising,” said Suleemas, whose flat is above the waterline on the second floor. By the time she and her neighbours received a mobile phone warning, it was too late, and the street-level water was waist-high.

The 1970s Khlong Chan flat compound in north-east Bangkok where Suleemas lives has become a symbol of the disaster. Photos have been shared across Thailand of the flats rising out of the brownish water, with the rooftops of parked cars barely peeking above the surface, although the authorities had managed to drain away the worst of it by Thursday.

Later in the week, during a brief respite in the rain, Suleemas said the water had receded somewhat. “People are so happy to see the sunshine,” she said. “Yet, the smell is getting stronger. So much debris and rubbish floating all over the place.”

Built on a vast floodplain of the Chao Phraya River, Bangkok, a megacity of millions, relies on a network of muddy canals to drain water, as well as pumps and dikes. But concrete construction has destroyed much of the canal network, and the city is unable to cope.

The government has declared a flood disaster across all 50 of the city’s districts. Some neighbourhoods have turned into dystopian scenes, with people using blow-up paddling pools or, in one case, a floating fridge, as boats to reach their homes. One man was arrested for riding a jetski along a main road.

“It’s a lesson,” said Chaleerat Chamrasporn, 55, a resident. “The kids are into doomsday-themed content, so now they can see what the ‘end of the world’ is actually like, or rather, that this is just the beginning.”

Flooding has also disrupted Bangkok’s main Suvarnabhumi airport, with about half ⁠of Thai Airways’ workforce unable to report for work at one point this week, causing thousands of items of luggage ​to pile up and forcing flight ​cancellations. Many schools have closed, and with much of the capital in gridlock even people in less affected areas have been stuck at home.

Thailand’s prime minister, Anutin Charnvirakul, told reporters that Bangkok’s metropolitan area was still at risk from flood waters flowing in from ​the north, and said a war room had ‌been set up to monitor ‌the situation. Thailand’s weather ⁠agency has forecast more ​heavy rain for next week.

It is part of a pattern that seems to be setting in this part of the world. South-east Asia has already seen a worsening of extreme floods, scientists say, a trend that is set to worsen as fossil fuel pollutants bake the planet further. Hot seas evaporate more water and warm air can hold more moisture, allowing downpours to crash down with far more force.

“In central Thailand, I can see from my maps, we have already seen a small but significant trend towards extreme rainfall,” said Nikolaos Skliris, an ocean scientist at the University of Southampton who has studied heavy rain in the region. “In the long-term, all models show this will be amplified – more rainfall, and more extreme rainfall.”

It is too early for scientists to attribute these particular rains to climate breakdown without doing a study. They were caused by a low-pressure system that moved from an unusually warm South China Sea over Cambodia and Vietnam before getting stuck above Bangkok, which was fairly cool, data from Copernicus Climate Pulse showed.

The picture is complicated by the emergence of an abnormally strong El Niño, a cyclical weather pattern that brings record heat and more violent extremes around the world. Its presence typically leads to drier summer conditions in south-east Asia but recent research suggests it still exacerbates days with extreme rainfall.

Naim Laeni, a consultant and part-time university lecturer who co-authored a study on Bangkok’s flood resilience, said authorities had underestimated the strength of the rains, which persisted for days. The city’s drainage canals can handle about 60mm of rain an hour, rising to 80mm in upgraded sections, but localised bursts exceeded this while leaving too little time between periods of stress.

“The government did not communicate the seriousness,” said Laeni. “Usually in Bangkok we receive heavy rain for a few hours, not for days.”

Neighbouring Myanmar has suffered from the same weather system, with flash floods and blocked roads complicating rescue efforts in the country.

Rescuers were pulling bodies from a landslide that devastated the village of Ka Det Nge Htein in Myanmar’s south at the weekend. That disaster had already claimed nine lives, and eight more people are missing under the mud in another village, local officials said.

Civil war across the country, and a junta that restricts the media, have made it hard to get information, and hampered aid organisations.

Back in Bangkok, pumping stations have been operating at full capacity, but it could be days before the waters fully drain.

At her apartment complex, Suleemas said she would stay where she was with her neighbours.

“It’s hard, yes, very hard,” she said. “But together we are fighting and biting through it. When we get together to help one another like this, our spirit can keep us on our feet; we can get through it all together.”

Agence France-Presse contributed to this report