Clouds of plague locusts are swarming across rural areas in the Australian state of New South Wales just as many farmers are preparing to harvest their crops.

One swarm in the Condobolin district has been measured as 6km long and 170m wide, according to the state's primary industries minister, Ian Macdonald.

Other swarms of the voracious insects, which are attracted to green vegetation, are threatening the Wagga, Gundagai and Narrandera districts. They are expected to begin laying another generation of eggs that could begin hatching in mid-December.

In some areas the crops that are due to be gathered are the first that farmers have managed to raise in several years, after many seasons of drought. Recent rainfall is producing an increase in grass and weeds along roads and at the edges of fields that could provide an extra food source for the locusts, and increase the number of eggs they are able to lay.

Selwyn Geddes, a sheep and crop farmer in Dookie whose farm lies in a locust hot spot, told the Country News website that he was too busy to spray the insects now that the harvest had begun.

"At one end of the paddock we have sheep and at the other we have hoppers," Mr Geddes said.

"I'm on the header now and don't have the time to spray. We need to get some assistance from the Department of Primary Industries to do an aerial spray."

Mr Geddes said DPI workers who visited his farm to confirm the sightings had been amazed by their numbers. "They're feeding on the green grass beside the roads and you can see them going across the roads," he said.

So far all are rated as low density swarms, but the state authorities have put nine planes on standby to attempt to disperse them in case they grow larger or thicker.

Field teams from the DPI are gathering samples of adult locusts, and monitoring the density of swarms.

"Reports of swarm activity are coming in to us from a number of areas of the state, particularly in the south and in the central west," said Mr Macdonald.

"A couple of these swarms have reached significant size, one of which is about six kilometres long and about 170 metres deep. This is quite a significant swarm.

"The government, in response, placed on standby nine aircraft ready to hit the swarms so that they don't become a major problem in parts of the state. The problem is that at this time, locusts seek out green fodder and so anything growing at this point of time would be attacked fairly severely by locusts."

Mr Macdonald said there was no need to panic. "It's important to realise that most of the state's crops are in the final stages of maturity and close to harvest, so are brown in colour," he said.

"Fortunately this means they are not as attractive to locusts, which prefer green plants, for example irrigated lucerne crops."

He urged farmers to report any new locust swarms to their local Rural Lands Protection Board.

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