Johannesburg, South Africa's largest city and economic hub, on Monday imposed emergency water restrictions as supplies deteriorated due to a drought, the worst to ravage the country in three decades.

"We are pleading with our customers to take more precautionary measures on how to use the water," the city's environmental affairs spokesman Anda Mbikwana told AFP.

The decision was prompted by a warning from Rand Water, the country's main water utility, early Monday that supplies were declining.

Residents have been asked to stop watering their gardens during the day to prevent evaporation, to re-use bath water and take shorter showers.

The drought has left farmers reeling in at least five of South Africa's nine provinces.

Among the worst affected areas is North West province, where dams are only carrying half of their normal capacities, with one dam, Molatedi, at only five percent capacity.