The Australian Plague Locust Commission and the Department of Primary Industries have mapped egg laying sites but will not know the full extent of where locusts will hatch until late next month.

According to the mapping, the estimated area of locust activity falls west of Ballarat, with several isolated locust sightings reported to June this year.

Plague locust commissioner Gordon Berg said farmers in the Ballarat area who had seen locusts in autumn should continue monitoring for eggs in those areas.

The government says locusts, which resemble grasshopers, can form large swarms and become ''eating machines'' and can quickly cause crop devastation.

Locusts can fly over large areas, putting all farming areas at risk.

Mr Berg said while Ballarat was outside of the main risk area, Bendigo had significant egg laying and if the city had an outbreak, locusts could migrate south.

''The situation will start to change once the eggs start hatching later in September through to October over the central areas and we're monitoring various sites,'' he said.

Mr Berg said weather predictions pointed to a warmer and wetter spring than average, which could bring the egg hatching dates closer.

Meanwhile, the state government unveiled an internet program this week to help farmers decide the best chemical to use against locusts.

It coincides with a list of chemicals released, which will be rebated under the 100 per cent rebate scheme for affected farmers.

The rebate scheme applies to eligible landholders shires which have the greatest number of egg beds, including the Pyrenees shire.