Standing waist-deep in brown floodwater outside her Bangkok home, Saisunee Sontana is short of food and getting desperate, while a short drive away air-conditioned restaurants serve well-heeled diners.

As a slow-moving mass of runoff water from the north creeps into the sprawling Thai capital, a stark divide is emerging in the metropolis of 12 million people, between the submerged suburbs and the bone-dry city centre.

Residents in affected areas complain their homes are being sacrificed to save downtown Bangkok's gleaming shopping malls, luxury hotels and the homes of the wealthy elite, triggering protests and the destruction of some dykes.

Saisunee's Bang Phlat district, on the western side of Bangkok's main Chao Phraya river, is one of the areas in the capital that is worst-hit by the floods, which have killed more than 400 people around the kingdom.

Filthy water has submerged roads and inundated the neighbourhood's small wooden houses for more than a week.

"The help didn't reach us because we are too far away," she said. "In two or three days, I will be out of rice, and I don't know how to get more food."

Trucks delivering emergency supplies do arrive daily at a bridge that connects Bang Phlat to the heart of Bangkok.

But to reach the trucks, locals have to trek through hundreds of metres (yards) of dirty water, dodging floating trash, dead fish and the occasional flip-flop or jerrycan.

And since the aid delivery hours are random, many residents in cut-off, inundated streets are struggling to stock up on essential goods.

Pramet Deerad, 47, wearing an orange life-vest, said the quality of life in Bang Phlat was "getting really bad".

"They are happy on the other side of the bridge, while here we are in a terrible situation. We want the authorities to know about us," he said, calling on authorities to clear the rubbish in the near-stagnant water.

Meanwhile on the other side of the Chao Phraya River, the biggest inconvenience for most residents in the city centre has been shortages of drinking water in the supermarkets after a flurry of panic buying.

Local officials blame a lack of funding, boats and staff for not reaching all those in need in flooded areas.

It is "an impossible mission" to hand out food and water three times a day to people who have refused to move to emergency shelters, said Bangkok Metropolitan Administration spokesman Jate Sopitpongstorn.

He defended efforts to spare Bangkok's economic and political "heart" by diverting the brunt of the runoff water to other, poorer parts of the capital, effectively sacrificing the homes of some to keep others dry.

"You can cut your hand but you have to save your heart," he said.

The growing tensions are a reminder of the fault line that runs through Thai society, more than a year after about 90 people died in an army crackdown on mass street protests demanding more democracy and equality.

The "Red Shirt" demonstrators were mostly loyal to ousted former premier Thaksin Shinawatra, whose sister Yingluck is now the prime minister.

Ironically, the people who voted for her Puea Thai party are suffering the most.

The crisis has again highlighted the class divide, with "the more wealthy established areas being protected at the expense of the outskirts," said political scientist Thitinan Pongsudhirak of Bangkok's Chulalongkorn University.

But the authorities are not abandoning poorer city-dwellers deliberately, he said, blaming Thailand's "slow-moving and ineffective" bureaucracy for the lack of assistance.

There is little hope of immediate relief for the struggling residents of Bang Phlat, where life has become unrecognisable from just a few days ago.

Saisunee's brother Manus Sontana, who needs to go the hospital regularly for kidney dialysis, is stuck in his house as his health problems prevent him from wading through the floodwaters.

"Have you found me a boat yet?" the 62-year-old yelled anxiously out the window to his sister.

But while resentment was growing among some, others said they understood efforts to save the centre.

"It's good that inner Bangkok is not flooded, that way they can still find food for us," said Sombat Chansawang, 42, who returns to his inundated house every day to feed his chickens, a rooster and a rabbit.

"Will you take my rabbit?" he pleaded with an AFP reporter, holding up the fluffy white creature. "There is no more grass to feed him."

Despite trying to put on a brave face, the father-of-three admitted the situation was growing increasingly dire.

"If you need anything, you have to leave. Medicine, food -- you have to go out and get it. If you stay here, you're just going to die."