Could the NSW drought lead to Sydney's water restrictions?

New South Wales has been 100 per cent drought declared.

As Sydney faces falling levels of the dam and concerns around its desalination plant, one expert believes that water restrictions should be presented sooner rather than later.

The city's water storage fell to 65 percent this week, almost a quarter from last year, because NSW continues to be under drought.

Should the Sydney storage drop to 60 percent, it is assumed that water supplies from the $ 1,800 million desalination plant will be added. At current rates, this can happen in a few months.

But the plant still has to recover from the damage caused by the tornado three years ago. It can not even be lit until December with several more months needed to reach the total production of drinking water.

This has led Professor Stuart Khan of the School of Civil and Environmental Engineering of the UNSW to suggest "level one" water restrictions at the 60 percent threshold, instead of the 50 percent demarcated trigger.

Level one water restrictions include limits to sprinklers, when garden hoses can be used and how vehicles are washed.

Sydney desalination plant.

AAP

"The rate of decline is extreme," Professor Khan told SBS News, adding that the last six months was "one of the fastest decreases" in the water supply of the city he had seen.

"We have not even started the summer," he said, a time when the city uses more water.

And the latest weather outlook from the Bureau of Meteorology forecasts drier than average conditions in most of Australia during September and October, with temperatures expected to be warmer than usual.

Professor Khan said he said submitting the water restrictions was a "reasonable substitute" for the desalination plant.

He said that when the restrictions were used for the last time in Sydney nine years ago, they saved as much water as the desalination plant would inject into the system.

"Obviously there is a clear and well-developed plan … [But] we need to be adaptive, "he said.

Warragamba Dam when it was filled by more than 90 percent.

Warragamba Dam when it was filled by more than 90 percent.

However, state WaterNSW has minimized concerns about water shortages in Sydney.

He said this week that "the water supply of Greater Sydney is large and safe, and contains enough water to meet the demand for at least two years, even in the very unlikely event that there are no entries."

"The WaterNSW projections confirm that there is already enough water in the warehouses to guarantee supply until at least mid-2020, and in some areas by the end of 2021, even under continuous drought," he said in a statement.

"Even under a repeat of the worst recorded drought, Sydney's storage levels would only fall to around 30 percent of capacity."

WaterNSW CEO David Harris said, "There are reasons for precautionary planning being carried out by the Government of NSW, WaterNSW and Sydney Water, but there is certainly no cause for concern."