Multiple flood warnings are current for vast swathes of NSW as heavy rain batters the state, where saturated soil and swollen rivers could lead to landslides.
State Emergency Service Assistant Commissioner Sean Kearns said there were 54 evacuation orders in place across the state and 16 evacuation warnings.
The SES received more than 2400 calls for help in the past 24 hours and undertook around 100 flood rescues, mostly in Sydney.
The east coast low that hit the Mid-North coast on Monday is moving south over the Hunter and Greater Sydney and would track into the Illawarra and South Coast on Tuesday.
"We are expecting to see widespread heavy rains today which will lead to river rises," Mr Kearns told Sydney radio 2GB on Tuesday.
"If we do issue an evacuation order take heed of that advice."
The SES ordered residents in multiple suburbs in Sydney's southwest along the Georges River to evacuate overnight - some without warning.
People in Camden were told to leave on Monday night after 80mm of rain fell and the Nepean River flooded.
"If you remain in the area you may be trapped without power, water and other essential services and it may be too dangerous to rescue you," the SES warned.
The SES later issued evacuation orders for Chipping Norton, Georges Hall, Picnic Point, Pleasure Point and Sandy Point as well as parts of Holsworthy, East Hills, Milperra, Moorebank and Warwick Farm.
Evacuation centres have been set up in Canley Vale and Menai.
People living in parts of Emu Plains, Penrith and Mulgoa in Sydney's far west were told to prepare to evacuate.
The Bureau of Meteorology has issued a severe weather warning for damaging winds and heavy rain in Greater Sydney, the Hunter, the Illawarra, South Coast, Central Tablelands, Southern Tablelands and parts of the Mid-North Coast, South West Slopes, Snowy Mountains and the ACT.
Intense rain could lead to dangerous and life-threatening flash-flooding with thunderstorms and six-hourly rainfall totals in excess of 150mm possible.
Damaging wind gusts peaking in excess of 90 km/h are also possible and storms are expected to continue until at least Wednesday.
Meanwhile, a 67-year-old woman and her 34-year-old son are missing after their car was found in a stormwater canal at Wentworthville in Sydney's west.
NSW Police were alerted after the Mazda3 was found in the Cooper Creek stormwater canal on Monday afternoon.
Sydney Trains is asking commuters to avoid non-essential travel and allow extra travel time as it undertakes inspections to ensure the rail network remains safe amid the deluge.
"What we are faced with is a very serious situation on broad areas of the NSW east coast," the BOM's Dean Narramore said on Monday.
The BOM warnings extend to "every community and town in between and extending well inland towards the Blue Mountains, Central Tablelands, Southern Highlands and possibly Canberra," he said.
A severe weather warning for damaging and hazardous surf is current for Hunter Coast, Sydney Coast, Illawarra Coast, Batemans Coast and Eden Coast.
Heavy rainfall over the past week has caused major flooding in the Hawkesbury-Nepean Valley.
The SES is warning river levels could rise along the Nepean and Hawkesbury rivers, potentially bringing "deep and dangerous flooding".
Major flooding is occurring at Menangle, North Richmond, Wisemans Ferry and Putty Road.
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