SOUTHERN Australia should brace itself for the worst locust plague in decades.

Ideal breeding conditions following excellent rain across inland Australia have swelled numbers of locusts and their good body condition means they are travelling large distances.

Fine weather this week will push adult locusts further south into Victoria, increasing the areas which where the locusts will need to be controlled in spring.

Australian Plague Locust Commission director Chris Adriaansen said there would be a "general southwards drift" of plague locusts into Victoria this week.

"The weather system shows two highs with a trough in between and this will mean a significant southwards drift of locusts into northern Victoria and southeastern South Australia, drawn from the Riverina," Mr Adriaansen said.

"The locusts are now very spread out, and some reports say that they are across an area the size of Spain."

Mr Adriaansen said the good seasonal conditions in much of inland Australia meant the adult locusts were in such good condition they were laying eggs, moving and then laying again.

"That has doubled the numbers (of eggs) and doubled the spread of where the locusts are," he said.

"We could be looking at one of the biggest populations of locusts for 30 to 40 years."

The problem with this latest locust plague is not the density but the spread.

The APLC is now working with state agencies to develop a plan of attack for spring.

"We need to make a lot of effort in the planning phase to make sure we are poised and ready to run flat out for six weeks," Mr Adriaansen said.

Already the locusts are making their presence felt on everything from early sown crops to sporting fixtures.

Earlier this month, officials called off a gallops meeting in Mildura and a commercial flight from the same centre was cancelled after a swarm of locusts descended near the airport.

Mr Adriaansen said there had been no official reports of crops being eaten by locusts, but he said any early sown cereals which were at the four or five-leaf stage would be "something adult locusts would look for".

The Department of Primary Industries reports locust have been seen as far south as Melbourne and Sale.

The department asks that locust sightings and especially egg laying be reported to the Victorian Locust Hotline on 1300 13 5559