Montagu was a town under siege on Monday night, its citizens trapped inside a ring of smashed bridges, their lives at risk from a giant dam that threatened to burst its wall.

Floods hit the Route 62 towns of Montagu, Barrydale, Ashton and Robertson at dawn on Monday.

And by the time night set in, the Little Karoo town of Montagu - only recently voted South Africa's favourite tourist village - looked set to become a full-blown disaster area.

Raging flood waters forced evacuations from low-lying areas across the town on Monday, and frightened residents fell asleep that night knowing that millions of litres more were rushing down the mountains towards them.

At dawn on Tuesday the Poortjieskloof Dam wall was holding in spite of the huge volume of water cascading over it.

At a high-level meeting of emergency service operators in nearby Robertson on Monday night, South African Air Force helicopter pilots reported on what they had seen on their last flight before dark: the Poortjieskloof Dam, about 25km upriver from Montagu, had filled dramatically since midday as mountain streams cascaded into it.

At 2.30pm it had been 6,5m from full. By 7.30pm it had begun to overflow.

"If that wall breaks, Montagu won't stand a chance," one farmer said.

The dam was so low a week ago that municipal officials were considering water restrictions for the area.

By nightfall on Monday night, sentries had been posted along the entire stretch of river armed with instructions to send an urgent alert when the next wave of floodwater arrived.

On duty through the night with them were 10 police divers and teams of paramedics from Metro Rescue, travelling to houses in low-lying areas and warning their occupants of the rising floodwaters to come.

Monday's floods had already dealt with Montagu's road network ruthlessly.

The first casualty was a bakkie which was swept away. Its two occupants were rescued, but the floodwaters made the town's four exits impassable.

The worst damage was on the Ashton road along the Kogmanskloof River, where two bridges looked from the air as if they had been bombed.

Most of those rescued by air on Monday had been trapped between flooded stretches of road.

About 10km downstream in Ashton, the river split the town in two, half of it now unreachable too.

Minutes before dark, Cape Argus staffers were aboard a convoy of three helicopters which swept through the mountain gorges to deliver Montagu its first aid - hundreds of loaves of bread and 100 blankets.

Worst-hit were residents of Montagu's tiny "RDP" (Reconstruction and Development Process) houses.

Huge cracks appeared in several of them as the rainwater seeped into homes up to 18cm deep. Much of the suburb was without water and electricity as a cold wind swirled along the swollen river. Dozens spent in four community halls.

Those areas of Montagu which were evacuated included the primary school, at least 20 houses, mainly along the town's historic Long Street, and the Avalon Springs Hotel, home of the town's famous hot springs.

Among the rescues during the day were two people off their rooftop and nine people and a dog trapped in a valley with the water rising fast around them.

Hotels and B&Bs reported cancellations as news of Montagu's natural disaster spread.

At the police station, several tourists were told there was no way out.

Dawn broke on Tuesday with relief that the Poortjieskloof Dam wall had held. But the town was still cut off from the rest of the world.

Although many of the roads in the Little Karoo have been re-opened, parts of Montagu and Ashton remain without drinking water, electricity or telephone lines.

"Our biggest challenge today is going to be the humanitarian aid," said emergency task force spokesman Colonel Piet Paxton this morning.

Four helicopters took off from Robertson's drenched airfield at first light on Tuesday, ferrying food and blankets to Montagu from Ashton.