Southern Mallee farmers have become the first in Australia to make an assault on mice with zinc phosphide.

Bait was dropped south-west of Birchip at Walkur last Wednesday, only a week after the National Registration Authority issued permits for farmers to use the poison.

And more than 250 Wimmerra/Mallee croppers have completed a two hour zinc phosphide course in preparation for ground baiting.

Twenty tonnes of bait have been distributed to rural suppy centres across the grain belt but mostly at Birchip - one of the state's worst mouse infested areas with densities exceeding 1000/ha.

Bait manufacturer Animal Technologies Victoria has produced 40t of bait and has enough chemical to make 100t, which would cover 100,000ha.

"It is difficult to tell what the level of demand for the bait really is but we have made enough to supply the existing need." managing director Dr Linton Staples said.

A small-scale manufacturing plant has been especially built near Kyneton but the exact location is being kept under wraps.

The zinc phosphide bait comprises the chemical combined with vegetable oil, which is coated to wheat that has been sterilised to prevent germination and spreading weeds.

The bait is applied at 1kg/ha and ground baiting is done with a special spreader.

Meanwhile, the CSIRO is back-pedalling slightly from predictions it made six months ago that a mouse plague was 80 per cent likely in the central Mallee.

CSIRO rodent research leader Grant Singleton said the dry conditions had helped keep mice numbers at bay. The most recent trappings in the central Mallee showed 300-400 mice/ha, at the lower end of the plague spectrum.

"Mice stopped breeding at the end of March, which is very good news for farmers because if they kept breeding for another three weeks, numbers could have doubled,"Mr Singleton said.

But he cautioned damage would depend on how hungry the mice were.