Dirty, tired and bedraggled, Imran beckons us over to the women who fled their village. They came 30 miles on foot only to spend almost three weeks here in the dead heat at this makeshift camp outside Sukkur in southern Pakistan.

"Take some photographs," he asks. "You sure this is okay?", I reply, our conversation translated from Sindhi to English and back again by Nizam Ud Din Bharchood, a long-time charity worker for Hands, a NGO based in southern Pakistan. "Go ahead, he insists," assured Nizam.

Under normal circumstances, foreigners cannot take photos of women or girls in Pakistan, but Imran waives this, showing a canny insight into how best to raise awareness to his people's plight. The ladies, adorned in their assorted pinks, greens and orange veils, clasp their children close and sit atop a rusted old bed, one of the few possessions they managed to carry from one of their houses.

It comes as a welcome counterpoint to comments attributed to the Pakistani Taliban that the presence of foreign aid workers is "unacceptable" in Pakistan. Other rules have been relaxed, with the Ramadan fast not compulsory for people affected by the disaster, according to Sikander, a volunteer who brought food to the roadside group.

Photo taken, Imran tells me more about their plight. "We are here 20 days now, without any shelter and only a little food." That is some ordeal, for women and small children. One woman pipes up, unsolicited: "Some people dropped off food here and we give thanks for that," she said. "But it was done in a disrespectful way – they just threw it off the back of a truck, like they were feeding animals."

Nizam says so many people need help and the logistical challenges are so formidable, it is impossible to meet needs even now, weeks since the Indus first spilled over the levees and onto people's land, over people's homes and into people's lives. "Many are angry and upset – that is why that lady is so unusually outspoken," he explains.

With well-established local links and networks in the vast, agriculture-dominated rural Sindh province, Hands is well-placed to help foreign humanitarian organisations such as Goal access where needs are greatest, quickly and effectively.

Goal emergency coordinator Brian Casey flew direct from Niger – where another food shortage looms – to manage the organisation's work in Sindh. Well-versed in disaster response, he compared the floods with calamities elsewhere in the past. "When you look at the figures, three million people were affected in both the Pakistan and Haiti earthquakes, while approximately five million people were affected across at least 10 countries in the 2004 tsunami. These floods have affected 20 million people in one country alone."

A clinic has been set up nearby, in Adhitakri, a rock-strewn wasteland 10 miles outside Sukkur. This is funded by Goal, with medical staff sourced locally by Hands. With cases of water-borne diseases such as cholera and typhoid on the rise, and people reporting skin infections and diarrhoea, a lack of healthcare threatens to undermine aid efforts in areas like food and shelter provision. The floods have destroyed 400 clinics or hospitals countrywide, so it is time for aid workers to get hands-on.

Later on, at Shahdad Kot, about a two-hour drive away, we meet volunteers from the Pakistan Fisherman's Forum (PFF), which says it has rescued and evacuated thousands of flood refugees, in operations across the vast flood waters. The town of Shahdad Kot itself is empty after an evacuation order was issued on Thursday morning, with water half-covering the mosque on the road leading in.

"We took 89 families from the villages and farms," says Jawad, a volunteer with PFF, as he points across the rice paddies to a spot on the horizon. Only now the cropland around the villages and farms is six feet under water, and we are standing on a hastily-piled dyke, made of rock, sand and topsoil.

At our feet the water cascades over the hard shoulder of what was a track through the farmland, along which the farmers would drive their gaudily-decorated tractors and carts, towed by donkey or camel. Now their only way out is by boat.

However, the water in the southern reaches of Sindh province – where the almost-2,000 mile Indus empties into the Arabian Sea – is continuing to rise. An evacuation order was issued on Thursday by the Pakistani government for 500,000 more people to vacate areas under new threat, just east of Karachi, as the river, seemingly-immovable, meets the irresistible sea, now in high tide. There is nowhere for the water to go, but sideways, over the dykes and levees, before covering more homes and land.

People here are resolute, however. All along the roads through the province, thousands are on the move. Many stop with their families wherever they can find a dry , sheltered location, while others are in camps run by the Pakistani authorities or by the UN and NGOs – many of whom are funded by USAid, whose logo is splashed on the rolls of plastic sheeting and tarpaulins being handed out as shelter, some comfort from the 40°C daytime heat.

For aid workers and concerned local officials, it is a massive challenge, given that so much infrastructure has been washed away. How to get shelter to people and/or how to get people to shelter? How to stop the spread of cholera, typhoid, malaria – while planning how to get four million homeless people back to their homes and land once the waters recede. It all brings to mind Martin Luther King's metaphor that "we must build dykes of courage to hold back the flood of fear".