NSW has been the state most devastated in the bushfires crisis this year.
"There are now hundreds of calls for assistance, including trees, boulders or power poles down onto cars and homes, and across roads, as well as power outages and localised flooding impacting various roads and traffic lights", she said.
Water from the Georges River was inundating Milperra and Liverpool in western Sydney in the early hours of Monday, the NSW SES said.
The advisory warned that power outages could result and advised people to "avoid being outside in forested areas and around trees and branches".
The heavy rains are also raising fears of runoff spilling into local reservoirs, contaminating the drinking water supply to major cities, including Sydney.
"The rain is good for business and farms as well as being really good for quenching some of these fires we've been dealing with for many, many months", he said.
The heavy rainfall has helped firefighters by dousing bush fires that have been raging for months, including the Currowan Fire in the Shoalhaven after which burned 499,621 hectares and destroyed 312 homes.
Bush fires have wreaked havoc in New South Wales since September.
According to The Guardian, the last time the city received more than 100 millimeters of rain in a day was back in November 2018.
Parts of northern and inland NSW, along with southern Queensland, have been in drought since 2016, severely reducing river and dam levels while also creating the tinder-dry conditions that have fuelled this season's deadly bushfires.
Australia's central bank governor on Friday said the fires would likely trim 0.2 percentage points off GDP growth in the December and March quarters.
The State Emergency Service (SES) said 200 people had been rescued after driving into floodwaters during NSW's three days of torrential rain.
The wildfires have killed more than 30 people to date and destroyed more than 12 million acres of land.
"There will be definitely rainfall, it will rain but not as intense as we have seen".
But Adam Morgan, a meteorologist at the Bom, said the rainfall totals in those areas were not expected to be "anything near as heavy as what we've seen over the past few days".
Sunday is forecast to be the heaviest day for rainfall across the region, with up to 70mm of rain is expected to fall in Byron, while between 20mm and 60mm of rain is forecast for Lismore.