The biggest floods in more than 20 years are predicted for north-western New South Wales as landholders moved stock and valuables on Monday.

The State Emergency Service (SES) said the floodwaters could isolate farms in the Bourke, Brewarrina and Central Darling shires.

The SES regional controller for the far west, Graham Craig, said flood levels could exceed those of the April 1990 floods and people may be isolated for up to five weeks.

"It's vitally important that they take the opportunity now to move their stock to higher ground to minimize the risk of those stock being trapped by the waters as they come down over the next week," he said.

Craig said extra SES operational management teams from across the state are set to arrive on Friday.

"We'll actually move both rescue personnel, aviation assets, as well as operations personnel into Bourke and Brewarrina," he said.

"Mainly for the personnel - the aviation assets will be marshalled at a closer location, not necessarily in those, until they're needed."