AUTHORITIES are urging people in parts of NSW to stockpile enough food for a week with fears they may become trapped by severe flooding.

The extreme weather is expected to hit parts of the state's central and northwestern regions on Christmas Day, meaning today will be the last chance for residents to reach the shops.

Bureau of Meteorology (BoM) forecasters have issued an alert about a developing weather system, formerly known as Tropical Cyclone Laurence.

The system is expected to track across the continent from Western Australia and become slow moving near the Queensland/NSW border tomorrow.

"This event could bring the heaviest widespread rainfall to northern inland NSW for over a decade,'' BoM NSW regional director Barry Hanstrum said.

BoM's regional hydrology manager Gordon McKay said the effects could be felt through to the state's far west.

"If predicted rainfall totals of 200-300mm eventuate, there could be major flooding in many areas including Gilgandra, Coonamble, Bourke and Nyngan,'' he said.

"Our main concern is flooding along the Castlereagh, Bogan, Lower Namoi and Lower Macquarie rivers, and a flood watch has been issued.

"It's too early to pin down exactly where the worst flooding will be, but I urge people to maintain a close watch on the latest forecasts and warnings from the Bureau of Meteorology.''

The NSW State Emergency Service (SES) has advised people in parts of central and northwestern NSW to stock up on supplies.

"Rural property owners in the affected areas should ensure that they have sufficient food, fuel and medicine to last five to seven days,'' SES commissioner Murray Kear said.

Livestock should be relocated to high ground where they can be provided with feed for the duration of any flooding.

Farm equipment and pumps should also be relocated to higher ground.

"Rain may make unsealed roads impassable and causeways may be covered in fast flowing floodwaters,'' Mr Kear said.

"The SES urges that people never drive, ride, walk or play in floodwaters as water may be deeper or faster flowing than it looks and contain hidden snags and debris."