'Airspace' still being created in the Murray River's full dams as the deluge approaches

Residents of New South Wales and Victoria are being warned of the biggest rain event in a year likely to produce up to 100mm of rain in some parts from tomorrow.

River authorities are "creating airspace" with releases from already full Murray River storages like Hume and Dartmouth dams.

Dartmouth Dam is still releasing water down its spectacular rock spillway, something that has not happened for 26 years.

Police are reiterating pleas for drivers to avoid flooded roads after the body of a man was found in his submerged car in NSW 's central west.

The death of the 46-year-old comes as much of NSW braces for further inundation, with more than 100 flood warnings in place across the south.

The Bureau of Meteorology says a trough approaching from the west is expected to bring thunderstorms to NSW and Victoria on Thursday and Friday.

Senior forecaster Jonathan How said from late Thursday into Friday, heavy rain would push into Victoria's northeast and across the western slopes of NSW.

The bureau this morning issued a severe weather warning for all areas of central Victoria and a wider region stretching 530km south from Mildura to Geelong.

MORE READING: SES says batten down the hatches.

They could be hit by heavy rainfall and flash flooding from now into Thursday.

"Six-hourly rainfall totals between 30 to 50 mm are likely, with isolated heavier falls of 65 mm. 24 hour totals during Thursday are expected to reach 50 to 70 mm, with isolated totals to 100 mm over higher terrain," the bureau said in its latest advice message.

"Heavy rainfall increases the potential for landslides and debris across roads".

This may cause renewed flooding along rivers in parts of the central west and southwest from Friday, many of which are already full due to heavy rain in recent months.

Last month was the fifth highest rainfall on record for Australia as a whole and above average for most of the country.