Dam levels in the Western Cape fell to 25.3 percent on Monday from 26.6 percent the previous week, and from nearly 40 percent a year ago, according to data from the department of water affairs.
City authorities have said residents will have to queue for water when levels drop to 13.5 percent, a figure expected to be reached in April.
Cape Town told residents last week they would need to cut their daily water consumption by almost half from next month as authorities scramble to prevent the city running out of water.
The water crisis poses a risk to a vibrant tourist industry that sees almost 2 million visitors flock to Cape Town every year.
The drought is also expected to hit wine production - as the Western Cape faces the smallest harvest in more than a decade.