Sydney's dam levels are falling so quickly the city would need the equivalent of a metre of rain and a major deluge to break the dry spell.

Details of the state of Sydney's catchments were circulated to government agencies this month as NSW braces for increasing stress on water supplies, particularly across the northern Murray-Darling Basin.

Warragamba Dam's storage has fallen below 50 per cent this month. Credit:Wolter Peeters

"It will take approximately 1000 millimetres of rain to fall over the course of a year in order to break the current drought in Greater Sydney, including an intense rain event of 200 millimetres over 1-2 days," the government document states. "The annual average is 850 millimetres."

Sydney's dams are 48 per cent full, down more than 10 billion litres over the past week. The weekly drop of 0.4 percentage points would have been more without the 250 million litres being produced daily by the city's desalination plant.