The heavy rain kept falling overnight and into Sunday morning as rivers across NSW and near Sydney overflowed or threatened to flood.
Residents in the Pitt Town Bottoms, Cornwallis and North Richmond areas west of Sydney were told to evacuate in the early hours as the Hawkesbury River began to flood.
The State Emergency Service issued several orders telling people in these areas to take pets and essential items with them and stay with family or friends, or go at an evacuation centre.
"If you remain in the area, you may become trapped without power, water and other essential services," the SES said in the evacuation orders.
Bellingen residents and people at a tourist park were also advised to evacuate because of the risk of flooding along the Bellinger River.
Evacuation centres were established at Richmond and Bellingen, adding to several others opened in the Mid North Coast and Hunter regions.
Flooding was also expected along the Nepean River and the SES urged people in low-lying areas to protect their homes by sandbagging doorways and clearing drains.
Farmers near the Colo River in the Blue Mountains were also told to be on alert for flooding and be ready to move livestock and equipment.
Sydney's main water source the Warragamba dam started spilling over on Saturday afternoon, causing river levels to rise along the Nepean and Hawkesbury rivers.
The heavy rain is expected to keep falling until Wednesday morning and people across the state are on high alert for rising floodwaters.
Premier Gladys Berejiklian said weather modelling predicted it could be a one-in-25-year rain event.
"This will be a deep-seated, extreme weather event," she told reporters.
"It's not going to be an easy week for us.
"None of us are out of the woods while the storm front is moving south."
The premier urged NSW residents who weren't in immediate danger to restrict their movements and heed the warnings.
Many areas across eastern NSW recorded more than 100mm of rain over the past 24 hours, particularly in the Blue Mountains. Picton received 159mm and Oakdale recorded 139mm.
A severe weather warning for intense rainfall and flooding remains in place covering the majority of the NSW population.
The Bureau of Meterology has issued flood warnings for more than a dozen rivers across the state.
"It's a very dynamic and evolving flood situation," BOM national flood services manager Justin Robinson said.
BOM says greater Sydney, the Mid North Coast and Central Tablelands areas will see more heavy rain on Sunday.
Parts of Port Macquarie and nearby towns have flooded and further south Taree is in the grip of a flood rivalling its worst on record nearly 100 years ago.
Emergency crews kept working through the night, having already made more than 500 floodwater rescues.
A bodyboarder in his 60s went missing off the coast of Coffs Harbour on Saturday afternoon and crews will resume the search on Sunday morning.
Strong winds have also caused damage, with a small tornado ripping through Chester Hill High School in Sydney's west.
The rain and foul weather is being caused by a coastal low-pressure trough combined with a strong high pressure system in the south.
Prime Minister Scott Morrison said the scenes across NSW were "absolutely heart-breaking" and the government was ready to assist.
The federal government has announced financial assistance for more than a dozen local government areas.
The COVID-19 vaccine rollout in Sydney and many parts of NSW has been delayed because of the extreme weather, the Department of Health said.
The Public Information and Inquiry Centre provides information about the severe weather at any time of day on 1800 227 228. For emergency help in floodwaters, call the SES on 132 500.
