Sydney residents have been urged to brace for flooding over the coming days with NSW Premier Dominic Perrottet saying "the worst is yet to come".
A severe weather warning is in place for heavy rain and high winds along the NSW coast from Newcastle to Bega, which could cause widespread damage and flooding.
The State Emergency Service says heavy to torrential rain is expected to hit Sydney, the Illawarra and the South Coast as an east coast low moves towards the state.
The Bureau of Meteorology (BOM) said the severe weather was likely to begin this evening, and some areas could record up to 200 millimetres of rain in a six-hour period.
It is expected to bring minor to major flooding to the Nepean and Hawkesbury rivers, affecting parts of Sydney's west and north west.
Dangerous flooding is also predicted along the Cooks River in Sydney's inner and south-west, while local flooding is expected on the Parramatta River and in the city's north and south.
Minor to major flooding is predicted for the Georges and Woronora rivers, which could affected areas including the Sutherland Shire and western suburbs.
Large swathes of northern NSW have been flooded in the past 48 hours, and as the weather moves south, so will the danger.
More than 1,000 flood rescues have taken place and 6,000 calls for assistance have been recorded, as the state's emergency services are stretched.
LIVE COVERAGE: Follow flood developments as they happen on the ABC's blogBut the Premier said the state must brace for further devastation, with Sydney soon to be in the firing line.
"The worst could be yet to come ... the flood rivers remain a real risk to people's lives and we will be doing everything we can to get every person, every family, every business back on their feet as quickly as possible," Mr Perrottet said.
"I also want to say as well, we know that at the moment this is focused on the north. But very quickly, as we're seeing in metropolitan Sydney now, it will move to the south.
“We say to people across our city and across our state, look after yourself, follow the advice, stay safe and we'll all get through."
Emergency Services Minister Steph Cooke said that for Sydney, "the time to prepare is now".
"As this event moves south, people in Sydney and on the South Coast have time," she said.
"We have seen what is unfolded in the northern parts of the state over the past few days. The time to prepare is now.
"Have an emergency services kit prepared with your medications, with your important documents, your laptop computers, mobile phones and chargers, so we can as best as possible keep track of where you are, that you can contact us if you need to, but more importantly, that you can be prepared to evacuate should the need arise."
As the wild weather travels south, Sydney's north-west areas could be the worst hit.
SES commissioner Carlene York said water had already been released from Warragamba Dam in anticipation of the heavy rain.
"Water New South Wales are in charge of the decisions to release water and they are releasing some water from that dam," Ms York said.
"What they have to do is assess the risk of increasing the chance of flooding downstream by releasing that water."
WaterNSW said in a statement that Sydney’s dam network was at 97.6 per cent storage capacity and Warragamba Dam at 99 per cent.
"Heavy rain forecast by the Bureau of Meteorology is likely to generate sufficient inflows to cause spill events in coming days at a number of dams, including Warragamba," the statement read.
BOM meteorologist Dean Narramore said there could be moderate flooding along the Hawksbury River into Richmond and Windsor.
The same area was impacted by devastating flooding a year ago.
Mr Narramore said isolated, heavy rain across Sydney could reach up to 200mm which could lead to flooding.
"We're very interested in the Hawkesbury and Nepean area," Mr Narramore said.
"We're expecting moderate flooding, and a watch for moderate to possibly major flooding, but definitely moderate at this stage."
Damaging winds could lead to costal erosion, power lines collapsing, land slips and mudslides.
Mr Narramore warned drivers to be cautious.
“Travel will be dangerous. Please be careful," he said.
The rain is expected to travel further south past Sydney by the end of the week.
