South Africa on Tuesday declared a "national disaster" over a drought that has ravaged parts of the country and threatened to leave the city of Cape Town without domestic tap water.

In a notice published in the government gazette, the department of cooperative governance said it had elevated the drought to a "national disaster" after reassessing its "magnitude and severity".

Cape Town is in the grip of a catastrophic three-year-long drought as sparse winter rains have failed to bring relief, and dam levels have dropped to dangerously low levels.

South Africa's second city is now facing the prospect of having to turn its taps off under a so-called "Day Zero" scenario to conserve the city's remaining water supplies.

On Tuesday, the predicted date for Day Zero was pushed back to June 11 after consumption cuts.

But the widespread drought has gripped large parts of the south and west of South Africa.

The statement said that declaring a national disaster would mean that the national government was responsible for tackling the issue and ensuring immediate relief efforts.