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Even with the added water, Sydney's catchment levels are continuing to decline at the rate of about half a percentage point a week. On Friday, storages were about 54.6 per cent full, or the lowest since 2011.

The decline has come despite the city's total water usage during the first four months of 2019 dropping about 8 per cent from a year earlier to 192.9 billion litres.

"We’ve had extremely low volumes of water entering our rivers and storages," a Sydney Water spokesman said.

"The last two years - since April 2017 - have seen some of the lowest inflows into our catchment dams since the early 1940s."

Hot, dry and windy

Stuart Khan, a professor at the University of NSW's School of Civil & Environmental Engineering, said a lack of rain over the catchments had been exacerbated by increased evaporation rates from warmer-than-usual temperatures and stronger winds.

Both Sydney and NSW as a whole had their hottest January-April periods on record, the Bureau of Meteorology said. Evaporation levels during the summer for NSW were highest since 1982-83.

Professor Khan said authorities are planning for level 1 water restrictions to kick in from about August - even with the desal plant operating at full tilt - assuming below-average rainfall continues and dam levels drop below 50 per cent full.

According to the 2017 Metropolitan Water Plan, the voluntary Water Wise Rules will become mandatory at that point. These include restricting the watering of gardens by hoses fitted with a trigger nozzle to before 10am and after 4pm.

Preliminary planning for the expansion of the desal plant will also begin - with construction beginning should dam levels sink to 35 per cent full.

A key challenge as Sydney's population continues to grow and residents have installed many of the water-saving devices - such as low-flow shower heads and trigger hoses - in response to previous public campaigns.

Per capita water use is about 200 litres a day, down from 250 before the Millennium Drought, Sydney Water says.

"[Authorities] don't think we have the capacity to have those big water savings this time," Professor Khan said. "We're not as wasteful as we used to be."

Sydney Water stepped up its advertising on Friday as part of its latest campaign to educate the public about drought after a survey by the agency found 47 per cent of residents did not realise the city was in drought.

"One of the surprising results of the research was the fact that more than a quarter - 26 per cent - of inner-city dwellers don’t believe the drought will impact them," Melinda Pavey, the NSW Water Minister, said.

"We’re experiencing some of the driest conditions on record and while we had some good rain in some areas during March, overall our dam levels continue to drop," she said.

"That’s why it’s never been more important for all of the Greater Sydney, Blue Mountains and Illawarra communities to come together to help save water."