Queensland police say they have uncovered one of the state's largest caches of illegal firearms and military-style weapons.

Three men have been charged over the haul, which police say included 328 weapons and 4.2 tonnes of ammunition.

The stockpile was found during a raid on a property near Monto, about 130 kilometres west of Bundaberg in southern Queensland.

Detective Superintendent Jon Wacker says local police, Australian Federal Police and Customs are all involved in the operation.

"We're still presently on the property. We've conducted a search of the property utilising SERT (Special Emergency Response Team), mounted police and other resources from the Australian Federal Police," he said.

"[The investigation] is being conducted with other agencies ... to establish if there is any criminal organisations involved with these weapons, and also to try to establish the origin of these firearms."

More of the guns seized in the Monto raid. ( Queensland Police )

It is alleged the firearms were stashed in sheds and other dwellings dispersed around an 800-hectare farm.

Detective Superintendent Wacker says of the 328 weapons located, the three persons were licensed to possess some of them, however the majority belong in a restricted category.

"We're still in the process of cataloguing those firearms. A number of those firearms have had modifications done to them," he said.

"The fully automatic weapons are military-style weapons. There is an MP5, AK-47s, they are fairly destructive.

"Virtually all weapons located are high-powered firearms."

Three men, including a 69-year-old man and his two sons aged 42 and 46, have been charged with unlawfully possessing weapons.

They will appear at the Biloela Magistrates Court on December 18.