Police say the body of a woman in her 80s was found inside a South Lismore home on Tuesday afternoon - the second elderly woman to have died in her flooded Lismore home.
Premier Dominic Perrottet will visit Grafton on Wednesday to speak with volunteers and be briefed about the floods engulfing northern NSW communities.
Ballina Hospital was evacuated on Tuesday night due to rising flood waters, with 55 general medical and rehabilitation patients, moved to Xavier Catholic College.
Just after midnight an evacuation order was issued for Ballina Island with the SES warning half a metre of flooding is expected, with the CBD expected to be inundated on Wednesday, isolating the community.
The Bureau of Meteorology has issued several flood warnings for the Greater Sydney area, including the Upper Nepean at Menangle, the Hawkesbury River at Richmond and Windsor as well as the Colo River at Upper Colo.
People in low-lying areas around Windsor and Pitt Town, north-west of Sydney have been warned to be prepared to evacuate.
The surface trough that developed into a low pressure system has already delivered hundreds of millimetres of torrential rain that has flooded southeast Queensland and northern NSW in recent days.
The slow-moving system arrived in Sydney late on Tuesday and the BOM is warning a deepening low pressure system off the Central Coast is expected to drift southwest towards the coast.
The system is forecast to hit central and southeastern districts during Wednesday and overnight into Thursday, with heavy rainfall over parts of the Hunter and Metropolitan, Illawarra, South Coast and parts of Central Tablelands and Southern Tablelands.
Six-hourly rainfall totals between 80 and 120 mm are likely.
The intense rainfall and thunderstorms could also cause life-threatening flash-flooding, while wind gusts could exceed 90km/h in some areas.
Warragamba Dam, west of Sydney has received 66.8mm of rain in the past week and is 99 per cent full and threatening to spill as heavy rain continues.
While the rain has eased in the Northern Rivers, the disaster is ongoing with some 35,000 people ordered to evacuate.
Some 22 members of the Victorian State Emergency Service and 12 South Australian SES members and fire service personnel are on their way to northern NSW to help with the disaster.
More than 20,000 residents in the region remained without power on Tuesday evening, however Essential Energy says it was able to restore services to almost 13,000 homes on Wednesday.
Teams are still unable to access many areas where floodwaters are yet to recede.
Police are urging bushwalkers to reconsider their plans after a man and woman were rescued in the Blue Mountains on Tuesday at Wentworth Falls, due to rising water on their walking track.
Lismore residents are no stranger to floods but the most recent inundation of the town has been worse than expected.
The Wilsons River at Lismore peaked near 14.4 metres on Monday afternoon after a levee constructed to mitigate flooding was overflowed by rising waters in the early hours of the morning.
Thousands have lost everything and are sheltering at evacuation centres while many are still unaccounted for because they were plucked to safety by civilians in a private flotilla of boats, kayaks and tinnies and weren't officially recorded by the SES.
Emergency Services Minister Steph Cooke said on Tuesday it would be "unrealistic" not to expect fatalities during a "disaster of this magnitude".
Seventeen councils in northern NSW have been officially declared disaster zones.
