A flood watch for the region was issued by the Bureau of Meteorology on Wednesday morning, as widespread flooding is possible for much of NSW from late Wednesday.

Major flooding is likely in the South West Slopes, it warned, and many catchments are already relatively wet from recent rainfall.

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"A trough of low pressure over western New South Wales will deepen to form a low pressure system on Thursday," the BoM said.

"This low will interact with very humid air to bring widespread rain to much of the state and heavy falls to the north."

Minor flooding is expected for Tumut River, Upper Murrumbidgee River to Burrinjuck Dam and Cooma Creek, and the Murrumbidgee River to Wagga.

BoM meteorologist Jane Golding said there is a real risk as the rain sets in across the state.

"We are looking at a wet and stormy second half of this week, we're already starting to see rainfall and we're expecting that rainfall to increase," she said.

"There's a real flood risk and dangerous flash flooding is on the cards."

The State Emergency Service has urged farmers to move pumps, animals and equipment to higher ground and prepare for roads to close.

Hazardous materials such as poisons, waste and chemicals should also be moved to higher storage locations.

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