Rain has eased slightly in NSW but is expected to return with renewed intensity as the north of the state is warned to brace for severe weather.
Several days of heavy rain which saw some areas receive more rain in 24 hours than would be expected for the entire month of February is expected to be followed by a low pressure system developing in south east Queensland moving south.
Intense localised falls are expected on Sunday, with thunderstorms a factor as well.
More than 100mm of rain is likely to fall in a matter of hours and could cause flash flooding on Sunday morning, the Bureau of Meteorology warns.
Some areas could receive up to 250mm in the afternoon on the Mid North coast and Northern Rivers region.
Residents in Lismore, Grafton, Coffs Harbour, Byron Bay, Ballina, Casino, Kyogle, Yamba, Maclean, Woolgoolga, Sawtell and Dorrigo are likely to be in the path on Sunday.
An evacuation warning has been issued in the Northern Rivers for residents in low-lying areas of Maclean, where the Clarence River is expected to flood properties and cut off evacuation routes.
The SES has warned people to stay off the roads unless necessary and not drive through floodwaters.
"It only takes 150mm of water to float the average car, and with flooding damaging most roads you don't know the condition under the water," the SES warned.
The SES has received 689 requests for assistance around NSW since noon on Friday and conducted four flood rescues in that time, with a total of 65 rescues since Tuesday.
People in areas expected to be impacted on Sunday should begin preparing if safe to do so, the SES says.
On Saturday, the heaviest falls were south of Sydney in the Illawarra, with 63mm falling at Bellambi in the five hours up to 2pm on Saturday.
About 31mm fell in the same period at Cronulla, in Sydney's south.
Pillar Valley, east of Grafton in the Northern Rivers, received 34mm.
The effects of the surface trough that's expected to develop into a low pressure system are already being felt in Queensland, with major flood warnings in place in the south east corner of the state.
Close to 700mm of rain fell in 24 hours up to Saturday morning at Mt Glorious, north west of Brisbane.
Four people have died in floodwaters in Qld this week, including an SES volunteer who lost their life on Friday night on the way to assist a family whose home was flooding.
The NSW SES has been in contact with its interstate counterparts to offer its condolences and says the volunteer's death "highlights the risks posed by floodwaters and the very real dangers SES volunteers face every time they attend a rescue".
"Any loss of this kind is deeply felt by the volunteer community," NSW SES said.
In NSW, a man died on the Central Coast on Friday morning after his Land Cruiser was swept away in floodwaters.
