The dams that supply water to the greater Sydney area have collectively reached their highest level in nearly two years.

Sydney registered its heaviest four-day rainfall in 30 years when 391.6mm fell in the city between 9am Thursday last week and 9am Monday this week. During this time, a rain gauge at Warragamba collected 399mm. 

This deluge caused large and rapid inflows into the city's drinking water catchment, allowing reservoirs to bounce back after nearly three years of steady declines. While the rain has now ended, inflows are still making their way into dams, causing storages to climb.

As of Tuesday morning, the Greater Sydney water storage level had reached 71.4 percent of its capacity. This is a 29.5 percent increase from the same time last week and the highest level since May 2018.

This overall storage level includes the reservoirs behind Warragamba Dam, which reached 69.3 percent of capacity on Tuesday morning, and Nepean Dam, which has now hit 100 percent.

Water NSW is taking precautions to avoid contamination of water quality from ash and debris being washed into storages from recently burnt out land.