An estimated 300 people have died in three months of torrential monsoon rains which have destroyed 400,000 homes, breached sewerage and freshwater canals and left two million people suffering from malaria, hepatitis and other sanitation-related diseases.
Three-quarters of a million people are living in temporary shelters and seven thousand people have been bitten by snakes in the water.
22 of the province's 24 districts have been flooded, ten of them severely, including several where millions of people were displaced just one year ago.
More than 21 million people fled their homes in the 2010 floods which left one fifth of Pakistan under water and killed an estimated 1000 people. According to aid agencies the impact of this year's flood is worse than last year because many of the victims have been forced abandon their homes for the second consecutive year, but a slow international relief effort is threatening to compound the suffering.
"Our opinion is that it's already worse than last year, not because of the numbers but the impact on a population already severely affected by last year's mega-flood," said Oxfam's country director for Pakistan, Neva Khan. "We're talking about the same population," she added.
An international relief effort, headed by Pakistan and the United Nations has yet to coordinate government aid and charities' responses, she said, but urged all individual donors and relief groups to help now before local conditions deteriorate further.
Ms Khan said the flood waters were rising and spreading throughout the country beyond Sindh province into Balochistan and parts of Khyber-Pukhtunkhwa, leaving new areas inaccessible to relief workers every day.
"We have more and more mosquitoes, the water is contaminated, and there's the risk of all the public health [diseases] because there is not sufficient clean water. It's not just about the world recognising it, but realising it is something we need to respond to now. People who have been displaced for a second time can't sleep at night. They're on the sides of the roads without shelter, suffering from diarrhoea, they're itching and scratching," she said.
Dr Safdar Abbasi, Sindh senator and former adviser to the late Pakistan prime minister Benazir Bhutto, said he believed the international community's "lethargic" response to this latest crisis was characterised by "compassion fatigue" after aid campaigns for victims of its 2005 earthquake and last year's floods.
In addition to affecting some districts affected by last year's floods, more than 1000 millimetres of rainfall in the last three months had devastated several districts relatively unscathed in 2010, he said. Last year, Badin district was a refuge for those displaced from neighbouring areas. This year more than 800,000 of its own people have been affected.
Ten provinces had been severely affected, Dr Abbasi said, with cotton, sugar, and chilli crops destroyed and herds of cattle swept away. "Three million have left their homes, smaller houses have caved in and in some villages the water is five feet high. The average annual rainfall in these districts is 150 millimetres per year, but we've had 1000 millimetres in the last three months," he said.
He said the poor international response had been matched by a patchy aid effort by the provincial and federal governments and a lack of trust in them on the part of western countries.
"Even for the NGOs who have performed well in previous crises, there's a fatigue syndrome," he said. "It's a lethargic response. I don't know if it is because of their economic problems, or if they have forgotten Pakistan," he said. He said most of his constituents whose lives had been destroyed in last year's flood had only received one fifth of the compensation payments they had been promised, and had been waiting on the balance to rebuild their homes.
Construction worker Anwer Mirani is one of 20,000 people in Jamshoro district who have been forced to flee their homes for the second time in 13 months. "We had just begun to restore our houses when we had to leave again because of the floods," he told Agence France Presse. He, his wife, parents and three children had escaped the rising water in a boat to the nearby hills. "What can we do except run away? No one can fight nature," he said.