It wants daily water usage to be reduced to 100 litres (26 gallons) per person.
Cape Town residents anxious about sediment from dams with low water levels are starting to boil tap water, said Justin Friedman, founder of For Love of Water, a non-profit group that promotes conservation.
"The disaster declaration will accelerate the Western Cape Disaster Management Centre's Project "Avoiding Day Zero", the Province's strategy to ensure that taps do not run dry", Premier Zille stressed.
The City of Cape Town's dam storage levels hit 20.7 percent this week, which is 0.7 percent down from a week ago.
South Africa's Western Cape region which includes Cape Town declared a drought disaster on Monday as the province battled its worst water shortages for 113 years.
The wealthy, able to buy water, are somewhat cushioned from the crisis, but the provincial government doesn't have enough water in its reservoirs to truck water to residents in parched townships. Zille's statement added that drilling into the Table Mountain aquifer will also take place.
The UN estimates that over 40 million people have been affected by the drought that was caused by the El Nino climate phenomenon.
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Government officials have warned residents not to flush toilets unless absolutely necessary, to use bathwater and dishwater to flush toilets, limit showers to two minutes or to give themselves sponge baths instead.
The city of almost 4 million people is in its low tourist season.
Some hotels provide place updates on the dam levels in guest rooms to encourage water economy. Other emergency interventions are under way and as dam levels decline, the City will start to implement a lifeline supply which entails reducing the water pressure to a very low level across the metro.
Cape Town in one of South Africa's main tourist destinations, but the peak of the tourist season comes later in the year.
The water crisis has forced the implementation of the provincial government's "Avoiding Day Zero" project, which includes a number of interventions that will be actioned nearly immediately.
This avoids a disaster during the dry months.