Roads have been cut off, with eighteen evacuation orders in western Sydney alone and warnings of more to come.
The area was hit by flash flooding in March, killing twenty people.
Emergency services minister for the state of New South Wales, Stephanie Cooke said it is a life-threatening emergency situation.
She said they are now facing dangers on multiple fronts flash flooding, riverine flooding and coastal erosion.
Her warning came as the Bureau of Meteorology said up to three hundred and fifty millimeter of rain had hit certain areas, risking flooding along the Nepean River.
