A man using a metal detector to search for treasure with his children has found a historic machine gun buried at Lake Hume, in Victoria's north.
The man then alerted police, who conducted a search of the area at Ebden Reserve and discovered three more firearms.
While the weapons are quite rusty, they are thought to be Thompson submachine guns, or Tommy guns, which were made famous by American gangsters in the 1920s.
Wodonga Detective Sergeant Graeme Simpfendorfer said the guns were missing some parts but could still hold some value.
"They're old and would be of great military significance, I would imagine," he said.
Gun dealer Ron Owen said the weapon, made in 1918, was later adopted by the American, British and Australian armies and was used in the Second World War.
"Britain bought around 200,000 from America in World War II to fill a gap," he said.
"They didn't have a submachine gun themselves."
The guns were recalled after the war when they were superseded by newer, easier-to-use models.
"Ammunition was hard to get, and the Owen gun was more reliable," Mr Owen said.
"The Thompson was quite involved — it had lots of little parts."
Could there be more out there?
Sergeant Simpfendorfer said the firearms were not suspected to have been involved in any crime, but police would investigate how the weapons washed up at Lake Hume.
"We'll work with some historians to try and identify the trace of this weapon and how they came to be here," he said.
Mr Owen said it was likely the firearms had been disposed of in the weir during the 1960s by the military.
"It's near Bandiana and that's where the strategic reserve was kept," he said.
"Lots of people would pull them out and try to resurrect them — it was quite popular in the '70s."
Mr Owen said said Tommy gun in perfect condition could be worth more than $3,000.
"It's a piece of history — it's an artefact," he said.
"To me it's as valuable as taking something from the Egyptian tombs."
