Cape Town will next month slash its individual daily water consumption limit by 40 percent to 50 litres, the mayor said Thursday, as the city battles its worst drought in a century.

Mayor Patricia de Lille has warned that if rains do not materialise and drastic consumption reductions are not achieved the normal water supply will be shut off.

Instead residents will have to queue at standpipes for daily water rations of 25 litres (6.6 US gallons).

"We have reached a point of no return... We can no longer ask people to stop wasting water -- we must force them," De Lille said as she announced a raft of tough new measures to fend-off the so-called "Day Zero" standpipe scenario, currently forecast for April 21.

A typical shower uses 15 litres per minute while a standard toilet consumes 15 litres per flush, according to WaterWise, a South African water usage awareness campaign.

One of the measures, which the council will vote on Friday, is a punitive tariff for the city's thirstiest consumers.

"Despite our urging for months, 60 percent of Capetonians are callously using more than 87 litres per day," said De Lille, referring to the current daily water consumption limit.

"At this point, we must assume that they will not change their behaviour," she said.

The city has rolled out a string of projects in recent months to increase its water reserves, including efforts to drill into aquifers and the construction of desalination plants.

But De Lille said those measures "will simply not be enough" and that the chance of reaching "Day Zero" was now "very likely".

"The crisis has reached a new severity necessitating a series of new emergency measures," she said.

The city, which attracts millions of tourists every year, has enforced strict waste controls including splash bans at municipal pools and hauling wasteful homeowners before the courts.

Water consumption in Cape Town has nearly halved since early 2016, but has remained stubbornly high at around 620 million litres per day -- 120 million litres above the city's target. "Day Zero" has crept forward by a week since the beginning of the year.

Mayor De Lille said the city would unveil 200 water collection points across the city next week "so that communities can begin preparing for that eventuality".

Strong summer rains saw much of southern Africa recover from a drought brought on by the El Nino weather phenomenon.

But Mediterranean-like Cape Town receives most of its rain in the southern hemisphere's winter -- and scientists warn there is no guarantee of a good rainy season.