An expert has warned Warragamba Dam is close to spilling over and causing an increased risk of flooding, as capacity nears 100 per cent following heavy rainfall.
The Warragamba Dam reached a low of 42 per cent in early February, but in the six months since, its capacity has more than doubled to 97.7 per cent full.
The dam, which is Sydney's main water source, has risen by almost 12 per cent in the last week alone and is now at its highest level since 2016.
Water NSW is making small controlled releases from the dam in co-ordinated efforts with the Bureau of Meteorology and the NSW State Emergency Service to reduce the pressure.
Parts of Western Sydney are notoriously flood-prone and if the dam is full, it reduces the capacity to hold back floodwaters.
Rain forecast for the end of the week is not expected to be heavy enough to cause the dam to spill over, but a substantial rain event could.
When it's full and it's forecast to rain, the dam is operated with a 0.3-1-metre buffer to avoid a large uncontrolled spill.
Meanwhile, the Nepean and Tallowa dams are already spilling over.
Professor Stuart Khan, from UNSW's School of Civil and Environmental Engineering, said this was cause for concern.
"At the moment we have effectively zero capacity to hold those floodwaters back so if we do get another big rainfall event and if we get significant flows into those catchments then we would expect to have flooding," he said.
"So I'm sure all the agencies — SES and others — are paying very close attention to that situation and preparing."
Parts of the dam are covered in a layer of ash and debris from the summer bushfires, which was pushed into the water supply following heavy rain in February.
Professor Khan said Sydney's drinking water would remain safe because water could be taken from deep within the dam, but it would have to be monitored closely to avoid any contamination.
"In 2007, at the end of the millennium drought, we had a big inflow of cold water during the winter time and that led to the growth of blue green algae on the top of Warragamba Dam," he said.
"And the previous time that happened was 1998 when the dam was about half full and filled up very, very quickly with a big wet weather event and that led to Sydney's notorious 'cryptosporidium giardia' incident."
