The hot and windy heatwave coupled with a lack of rain in Gauteng is contributing to the rapid decline in the level of the Vaal Dam.

The city of Johannesburg said on Monday the dam's levels had fallen below 50% for the first time in nearly three years.

VBHydrology and Gauteng Weather tweeted that the dam was only 49.64% full.

Residents are being urged to use water sparingly.

The department of water affairs said last week that the Vaal Dam, which is one of the critical dams in the Integrated Vaal River System, was at 51.5%, down from 53% the previous week.

At the same time last year, the dam was more than 86% full.

"Gauteng province is presently experiencing high temperatures (with maximums of about 30ºC) and this situation is compounded by the slight rainfalls which are scarce and far apart. Another contributory factor is the shutdown of the Lesotho Highlands Water tunnel for two months for maintenance (until the end of November)," the department said.

In contrast, the Sterkfontein Dam was holding steady at just over 90%, the department said. The Bloemhof Dam was also in a healthy state at 84%.

In the Western Cape the latest average dam level for the province is 64%, and 79% for major dams supplying Cape Town.

The situation in the interior of the province, including in the Karoo and particularly in the agriculture sector, remains serious and continues to be monitored, said the province.