Rain forecast for this weekend is likely to take Burrendong Dam to 100 per cent of capacity, but if need be it could store the contents of another "Sydney Harbour".
The Bureau of Meteorology is forecasting above- median rainfall for NSW this spring along with below-median maximum temperatures "west of the divide".
The dam's imminent milestone follows "the worst drought in living memory".
At the start of 2020, the giant of a dam was on its knees.
At less than 1.5 per cent of capacity, preparations were being made to pump its remnant storage water for the first time and take water from Windemere Dam.
But the heavens opened and Burrendong Dam began to find its feet.
WaterNSW says the dam's design and location makes it "very effective" at capturing water from its catchment of almost 14,000 square kilometres.
On Wednesday, it held 1170 gigalitres (GL) or more than twice the volume of Sydney Harbour.
"Multiple rain events in the Burrendong catchment in August generated inflows that increased the storage from 80 per cent of capacity to the current 98.2 per cent," a spokesman said.
"In the nine days to September 1, Burrendong received 140 GL, increasing the storage level from 87 per cent to 98.2 per cent.
"Even without further rainfall and inflows Burrendong storage would reach full supply level of 100 per cent by mid-September.
"However, with the Bureau of Meteorology forecasting rain for the coming weekend that 100 per cent mark will likely be reached sooner."
The last time the dam reached 100 per cent of capacity was in January 2017.
The WaterNSW spokesman said the dam's resurgence "translates to the best water security enjoyed by communities, farmers and the environment in the Macquarie Valley for at least half a decade".
Should the rain keep falling on its currently soggy catchment, the dam would cope.
WaterNSW confirms the dam can reach 142 per cent of capacity.
It has a flood mitigation zone which can hold about 490GL or "an extra Sydney Harbour of water".
"The conservation storage of 100 per cent is when the water level just reaches the bottom of the spillway gates," the spokesman said.
"The spillway gates are six-metres high and this allows for the storage of the extra 490 GL of water in the flood mitigation zone."
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