The state government has said that it had spent tens of millions of dollars to ensure that Adelaide, the country's fifth largest city, and the state had enough water.
State Premier Mike Rann described it as a "prudent and sensible" measure.
Drought has become a regular occurrence in South Australia, which receives the least rainfall of any Australian state.
Lack of rainfall and a sharp reduction in the amount of water flowing into the Murray River meant the state could not guarantee water levels for 2009.
The state's water security minister, Karlene Maywald, said she had purchased 61 billion gallons (231 gigaliters) of extra water for 2009.
Some of it has come from shared water resources with New South Wales and Victoria states, while the rest was purchased on the water trading market, she said.
Water levels in the vitally important Murray Darling basin - which produces 40% of the country's fruit, vegetables and grain - have fallen to critically low levels.
Ms Maywald said the region had sufficient water needs for this year, but had not yet accumulated enough for 2009-10. "So we've made a decision to go into the market and purchase water on the temporary market," she said.
Defending his Minister's move Rann said "We've been buying water for years, but this is water that's being bought as back-up water for critical human needs." "I think what she's done is absolutely prudent and sensible in the worst drought in a thousand years according to the scientific experts" ABC news quotes him as saying.