Tallowa Dam is also spilling, into the Shoalhaven River, with flows contributing to the highest flood levels downstream at Nowra in 29 years.
At its peak spill rate on Monday, Tallowa was releasing water at the rate of 375 billion litres a day, WaterNSW said.
The near-full capacity comes just six months after the storages dropped towards 40 per cent before a huge three-day rain event in February doubled water levels. The jump in inflows allowed the Berejiklian government to ease water restrictions and delay plans to double the size of Sydney’s desalination plant.
A spokesman for WaterNSW said Warragamba was not expected to spill as a result of current inflows generated by the rain event.
“However WaterNSW will be making small operational releases from the dam’s spillway gates in order to bring the storage back to target level in anticipation of further rainfall,” he said.
The forecast rainfall for the coming weekend had been scaled back, including 5 to 15 millimetres for Friday, but those predictions could change, the spokesman said.
Bureau of Meteorology forecaster Olenka Duma said “we’re not expecting a huge amount of rain” from the strong cold front that will move eastwards towards the end of the week. Still, there remains the prospect of thunderstorms over much of NSW as the front draws in tropical moisture from the north.
In addition, the bureau is putting the odds of a wetter-than-normal September-to-November period for the eastern half of mainland Australia at greater than 65 per cent.
Stuart Khan, a professor in the University of NSW’s School of Civil and Environmental Engineering, said the rapid shift from water restrictions to full storage “is something we haven’t seen since 1998”.
Professor Khan said it was “pretty likely” Warragamba would start to spill soon. The Wingecarribee Reservoir, for instance, was 99.6 per cent full, and any spill from there would largely end up in Warragamba via the Wollondilly River.
The rapid rise inflows has meant NSW Water Minister Melinda Pavey had been vindicated in her decision to stop work on preparations to double Sydney’s desalination plant, he said. The high flows, though, should not put a halt to consideration of water-saving measures such as water recycling.
“It’s exactly the time we should be talking about long-term water supply strategies,” Professor Khan said.
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Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.