Authorities in Sao Paulo, Brazil's richest state and economic hub, have warned they are considering severe water rationing if the country's worst drought in 80 years continues.

Officials outlined draconian plans for alternating cuts that would leave areas without water for five days at a time.

"If the rain persists in not falling into the Cantareira reservoir system, the solution would be for very heavy rationing," said Paulo Massato, director of the state water company Sabesp.

Sabesp runs the Cantareira system, which supplies nearly half of the Sao Paulo metropolitan area, South America's largest city with some 20 million people.

"The rationing would see two days with water and then five without," he said late Tuesday.

Massato said it was the only way to avoid a total cut in water supplies, which have been depleted by months of severe drought that have sent water levels to their lowest levels in living memory.

Unless it rains soon, supplies could run out altogether by March.

With around 40 million inhabitants, Sao Paulo state is an industrial behemoth that relies on large scale water supplies to run its plants and factories.

Recent weeks have seen growing protests by families complaining that they have had to go without water for days at a time as their their home taps run dry.

After long downplaying the prospect of rationing, Sao Paulo state governor Geraldo Alckmin conceded a fortnight ago that some rationing had already been introduced.

Alckmin has been the target of criticism for not starting rationing earlier.

The state government has been racing against time to step up attempts to finish projects running well behind schedule to link the Cantareira with other reservoirs that could eventually feed it with water.