Raging fires and blistering heat paralysed the Western Cape on Thursday, prompting a province-wide "red alert" of maximum readiness among the entire rescue fraternity.

A sweltering 40°C was predicted for Paarl on Thursday, with the city only slightly cooler at 38°C.

In an ominous warning on Wednesday, a senior nature expert said the fire that has burned for the past month above Somerset West, Stellenbosch and the Elgin Valley could have a severe impact on Cape Town's water supplies next summer as the Hottentots Holland mountains are the main provider of water to the Cape Metropolitan area.

This morning at least three major fires burned fiercely. First is the week-old fire in the Hottentots Holland Nature Reserve, which stretches between Franschhoek and Sir Lowry's Pass. The second is a new blaze at the foot of Sir Lowry's Pass in Elandskloof. And a third fire broke out late last night on the mountain above Paarl.

Between them, the three fires smothered greater Cape Town in thick dusty-grey smoke on Thursday.

A fourth fire was reported today, north of Malmesbury.

On Wednesday firefighters deep in the Hottentots Holland mountains had to be rescued by a helicopter after they were trapped by two fires closing in on one another.

"We can't afford to endanger lives," said Leon Lourens from the reserve's head office at Nuweberg outside Grabouw.

These were teams which had been fighting the fires on foot in the mountainous reserve, in areas inaccessible by vehicle.

The crews work 12-hour shifts, sustained by army food rations.

At 9am today, Lourens reported: "The fire's still raging out of control. It's still running towards Jonkershoek and Stellenbosch."

In Jonkershoek, nature reserve manager Patrick Shone reported that the fire had arrived in the valley. He did not provide further information as he was frantically directing firefighters.

CapeNature said Wednesday that additional firefighting teams from the northern part of the country were en route to the Western Cape and that teams from the Free State were already here.

One source said some of these up-country crews were battling in unfamiliar conditions.

"They're surrounded by mountains, by fires burning towards them from opposite directions, in high winds, updraughts, downdraughts in the kloofs, in thick smoke ... it's confusing if you don't know these fynbos fires," the source said.

Fanie Bekker, CapeNature's executive director of operations, ex-plained that when such large areas of catchments burned, "especially under extreme weather conditions and when alien vegetation is present, it may have a significant detrimental impact on soil structure and its water retention potential".

"This may lead to a significant reduction in the potential of our catchment areas to produce a sustainable flow of water with an acceptable quality," he said.

"The subsequent succession of alien vegetation proliferation after these fires may also lead to a significant reduction in water run-off from these areas, something the Western Cape with its water security challenges can barely afford."

Bekker said the fire damage could potentially reduce water run-off by "up to 50 percent or more".

"The cost of the current fire is not just extreme in terms of monetary and ecological damage, but psychological as well," he said.

At Paarl on Thursday, a staffer at a luxury hotel on the slopes of the Paarl mountain reported that the fire was burning on the north and southern ends of the mountain, but that the crucial eastern flank above the town remained fire-free.

It will be marginally cooler over the weekend, with 29°C forecast for Saturday and Sunday - mercifully for the thousands of cyclists who plan to pedal the Cape Argus Pick n Pay Cycle Tour on Sunday.

Cyclists on the Giro del Capo are likely to suffer in the sweltering heat today, as they race their second leg from Durbanville over 143km.