From northern central Victoria through to the Riverina in NSW, high-density hatchings have been observed in recent weeks and there is the potential for damaging swarms to form next month, the Australian Plague Locust Commission warns.

Without adequate reporting and chemical spraying, "you could have one of those biblical proportion-type swarms that cause massive damage across the state", said the NSW Minister for Primary Industries, Ian Macdonald.

The locusts are still too young to form flying swarms but they have started to bunch into large dense bands on the ground.

Aerial surveillance over the past week has alarmed authorities because numerous bands were observed that had not been reported by farmers.

Richard Le Lievre, a ranger with the Wagga Rural Lands Protection Board, has been spotting bands more than a kilometre long from the air.

Locusts can be sprayed effectively and cheaply by landholders while they are in bands on the ground, but aerial spraying of swarms once they are on the wing is expensive and difficult.

The insects form swarms of up to 25 square kilometres that can travel 500 kilometres a day searching for food.

A swarm of one square kilometre can contain 50 million locusts consuming up to 10 tonnes of vegetation every 24 hours.

Even at the nymph stage, bands of locusts can crawl up to a kilometre from their egg beds and consume the crops and pasture in their path.

Locust command posts have been established at Wagga, Forbes and Narrandera, while rural lands protection boards are distributing chemicals to farmers across the affected regions.

By yesterday 466 reports had been verified, with the Wagga district boasting 204 of them.

Mr Macdonald said: "Fifty-nine large bands of locusts have been found in the last 24 hours. They were spotted by planes between Narrandera, Grong Grong and Ardlethan. The largest of these bands covers around 24 square kilometres - so it's a massive area."

"There is enough chemical available for the estimated worst case scenario of 31,000 hectares and we have contingencies in place to get more chemical if required.

"If under-reporting continues, we could have an outbreak as big as spring 2004."

That year, locusts swept across inland NSW and laid eggs after good autumn rain in the north-west of the state and western Queensland.

Those eggs hatched in the spring, creating what was described as the worst locust plague in 25 years.