Burrendong Dam had risen to 99.54 per cent by 9am Sunday and it looks likely to fill - an incredible milestone following "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.
By last 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."
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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."
Water NSW has been releasing water from Wyangala into the Lachlan River to make room for the above-average rains forecast this Spring.
They had reduced releases in anticipation of significant rain generating inflows in the catchment's tributaries over the weekend after reducing the catchment to 94.5 per cent of capacity.
At this stage it's expected flood levels will be kept at the minor to moderate levels in Forbes and the surrounding river communities and numerous rural roads are impacted by water.
After an inch of rain on the weekend, the Bureau of Meteorology and State Emergency Service on Monday predicted the Lachlan River would peak at 9m at the Iron Bridge in Forbes on Tuesday. That remains within the minor flood level.
It's expected to reach 7.6m at Jemalong Weir with moderate flooding on Wednesday, and 5.3m at Cottons Weir also with moderate flooding.
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