Woronora Dam to drop below 40 per cent capacity by the end of the month

Woronora Dam in 2005 (top) at less than 50 per cent capacity and the dam in 2016 at 96.4 per cent capacity. The current dam level is 40.9 per cent.

Woronora Dam in 2005 (top) at less than 50 per cent capacity and the dam in 2016 at 96.4 per cent capacity. The current dam level is 40.9 per cent.

The dams serving Sydney are likely to sink below 50 per cent full for the first time in 15 years over the weekend.

Sydney's storages dropped to 50.1 per cent capacity on Thursday, and are losing 0.4 percentage points per week.

Woronora Dam is 40.9 per cent full while Avon Dam, also in the Woronora catchment, is sitting at just below 50 per cent full - about where the overall storages for the city sit.

Woronora Dam will slip to below 40 per cent capacity by the end of the month if the present slide continues.

Long-range weather forecasts suggest the slide could accelerate with little prospects for more than the odd shower in coming weeks.

Coastal showers have bumped up Sydney's winter rainfall tallies but these have virtually dried up in July and August. Inland regions, such as the city's catchments, have been drier still.

The Bureau of Meteorology's three-monthly outlook for the September-November period suggests the relatively dry times will continue for most of the nation, including most of NSW.

The key climate influence is from the Indian Ocean, where conditions off north-western Australia favour reduced moisture flows. Such a pattern typically produces a drier than normal spring for south-eastern Australia.

Avon Dam in the Woronora catchment area is sitting at just below 50 per cent full. Picture: Janie Barrett

Avon Dam in the Woronora catchment area is sitting at just below 50 per cent full. Picture: Janie Barrett

The odds also suggest day-time temperatures for most of the continent will be warmer than average for the September-November stint.

The last time Sydney's dams dropped below the 50 per cent was in May 2004, during the Millennium drought of the mid-2000s.

According to a WaterNSW spokesman, the rate of the present slide in storage levels continues to exceed the pace of that dry spell more than a decade ago.

The decline continues even though Sydney's desalination plant began producing water for the city's users in March and reached full capacity of about 15 per cent of total demand at the start of August, a spokesman said.

"Preliminary expansion planning" has now begun on doubling the plant's capacity of 250 million litres of water a day, he said.

Level 1 water restrictions were introduced on June 1, two months early and for the first time in almost a decade.

Sydneysiders have responded to restrictions, with usage about 7 per cent lower since July than forecast, a Sydney Water spokesman said.

Restrictions include garden hoses must be fitted with a trigger nozzle or similar attachment, and lawns and gardens cannot be watered between 10am and 4pm.

Warmer conditions and longer days will likely see evaporation rates increase. So far this month, Sydney's Observatory Hill has collected 3.2 millimetres of rain while evaporation was about 20 times that.

Sydney can expect to reach 25 degrees on Friday, about 7 degrees above the August norm.

The best chance for rain for Sydney in the next week may be on Monday but even then it is just a 15 per cent probability, forecasters say.