Sydney,  Australia's Bureau of Meteorology (BoM) on Friday warned of heavy rainfall across the state of New South Wales (NSW) from Saturday, with potential to cause major flooding into next week.

Senior meteorologist Dean Narramore told the national broadcaster ABC that the downpour would be focused around Sydney, and along the coast to the north and south, reports Xinhua news agency.

"We're looking at maybe a month's worth of rain in a few days," he said, saying areas would likely experience between 100 to 200 mm over the five days from Saturday.

"Over the next four days significant rainfall is expected in parts of NSW. This includes the chance of consecutive days of heavy rainfall, which is expected to lead to some river rises including minor to major flooding," the BoM NSW said in an announcement on Friday.

The Bureau warned that the low-pressure, trough system coupled with the state's water storages being at or near capacity could lead to sudden floods in many of the state's river systems.

"This Flood Watch means that people living or working along rivers and streams must monitor the latest weather forecasts and warnings and be ready to move to higher ground should flooding develop," read the notice.

The NSW State Emergency Service urged residents to prepare for the wet weather by putting in place an emergency plan, and preparing an emergency kit.

They also warned of driving through the wet conditions, which has been a major cause of fatality in previous floods as cars were swept away in flash floods.