In a statement sent to Business Insider, Cape Town's executive deputy mayor Alderman Ian Neilson noted that water consumption in the city dropped from one billion litres of water per day in 2016, to 830 million litres per day in 2017, to 526 million litres per day today.
The decision came as Cape Town announced its water saving measures, which require each citizen to use less than 50 litres a day, had successfully pushed back "Day Zero" to 4 June.
The decision was made after "a reassessment of the extent and severity of the current drought".
However, the decision to declare a national disaster means the central government - which is run by the African National Congress (ANC) - will now take responsibility for relief efforts. A five-minute shower uses around 45 litres.
It will probably arrive in May, a month or two before the onset of the winter rains.More news: Florida shooter booked, charged with 17 counts of premeditated murder
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However, Cape Town, whose picturesque oceanfront location is a major tourist draw, has pushed back to June 4 from May 11 its designated "Day Zero", when residents will have to start queuing for water, with city officials citing a decline in water usage.
Speaking to Al Rai Al A'am TV program, the minister said the drought crisis in South Africa did not emerge all of a sudden for there had been several repeated warning signs for many years.
DA leader Mmusi Maimane praised Cape Town residents for significantly slashing their water consumption.
The drought facing Cape Town is the worst in 100 years.