In autumn, mice attacked 3 million hectares of crops in NSW's central west and the Riverina, as well as parts of Victoria and South Australia.

Now there are fears rodents will eat their way through the bumper wheat, barley and canola harvests.

The NSW Department of Primary Industries director of invasive species, John Tracey, confirmed there had been more mice and crop damage in southern NSW since the start of spring.

''Ideal seasonal conditions and a large carry-over population from autumn point to a continuation of Australia's worst mouse plague in living memory,'' Mr Tracey said.

Peter O'Shannassy, a senior ranger at the Riverina Livestock Health and Pest Authority, agreed there were ideal conditions for mice to thrive.

''We haven't had the climatic conditions that could kill mice,'' he said. ''We haven't had extreme cold and there's tonnes of good food around.''

Simon Humphrys, from the Invasive Animals Co-operative Research Centre and chairman of the National Mouse Management Working Group, said research was under way into mice numbers and breeding patterns at four spots in NSW - near Dubbo and around Hillston, Griffith and Coleambally.

Dr Humphrys said farmers had suffered $200 million in crop losses across 3 million hectares in autumn, but that figure could have jumped to as high as $300 million during the spring mouse outbreak.

''There have been about 400 to 500 tonnes of bait [grain laced with poisonous zinc phosphide] used this spring to control mice damage.''

Ian Singleton, a grower from Jerilderie in southern NSW, has budgeted to lose about 15 per cent of his 450-hectare wheat crop to mice. He expects to lose 10 per cent of his canola harvest.

This is the most mouse activity Mr Singleton has seen on his property, Red Plain, since he began farming it eight years ago.

''The mice are coming along and chewing part of the way up the wheat stalk and they fell it like a tree to get to the grain, into the wheat head,'' Mr Singleton said.