As Hawkesbury bunkers down amid heavy rain, Hawkesbury NSW SES Unit Commander Kevin Jones said water could get "very close to the bridges by tonight".
"We're not really sure what's going to happen yet. They're [the Bureau of Meteorology] saying minor to moderate flooding. Moderate can mean it gets near the bridges, so we may or may not have the bridges closed by tonight, depending on the rain," Mr Jones said.
"We're waiting on more accurate predictions from the Bureau of Meteorology (BOM)."
Recent rainfall over the Nepean and Hawkesbury River catchments and a deepening East Coast Low is bringing heavy rainfall from Wednesday and has the potential to cause minor to major flooding along the Nepean and and Hawkesbury Rivers.
The BOM said this morning that major flooding is possible at North Richmond.
Minor flooding is currently occurring at North Richmond and Putty Roads.
Minor flooding is occurring along the Hawkesbury River at North Richmond. Moderate flooding is possible from Wednesday afternoon with further rises to the major flood level possible with forecast rainfall in the next 24 hours.
Minor flooding is occurring along the Colo River at Putty Road.
Warragamba Dam commenced spilling around 6am Wednesday.
A flood watch is current for the Upper Coxs and Macdonald River.
Mr Jones said the local SES had set-up equipment on both sides of the Hawkesbury River and were standing by as rain continued across the Sydney catchment.
He said if the water rises higher than the moderate flood level then the SES would close the bridges.
"We're actively ready to do whatever needs to be done," he said.
"We're currently dealing with lots of people who have leaking roofs , which is normal in this weather.
"We're getting our equipment and people ready in the right places."
He said, despite his experience, he couldn't tell yet if the flood would close the bridges.
He asked people to stay home if they didn't need to go out, and reminded people not to drive or walk through flood water.
He also asked people calling the SES number on 132 500 to be patient, as wild weather across the state meant there were delays on the line. Some reporting waiting on the line for over an hour before they received a response.
"Please be patient, and if you have a life-threatening problem, then call triple-0," Me Jones said.
Meanwhile, the local SES is establishing a sandbagging unit and will keep the community updated on where they can pick up sandbags via its Facebook page.