Police are investigating the hacking of a gun club database that may have exposed where more than 1500 semi-automatic handguns are stored.
The private details of 540 members from the Port Melbourne club, including the types of weapons they owned, is believed to have been compromised this month, potentially exposing them to the theft of guns worth at least $5000 each on the black market.
Most members own multiple guns, and store them in their homes.
One Melbourne International Shooting Club member who stores nine handguns in his home, including a 9mm Glock and Colt .45 automatic - both highly sought-after weapons in the underworld - urged police to confirm whether he was at risk.
"Criminals would see those guns as quite valuable," he said. "I fear for the safety of my family should this information get into the wrong hands."
Most members own at least three guns, another member said, estimating that as many as 3000 could be registered in the club database.
Police are investigating what was accessed during the breach, and will not comment on how many gun clubs have similar databases, how they are regulated, and whether the existence of such databases undermines the integrity of efforts to crack down on the spread of illegal firearms.
It is unclear how many guns have details stored in similar databases, and police declined to confirm how often these databases were audited for operational reasons.
"Inquiries are being made and it would be inappropriate to make further comment," a police spokeswoman said.
She said the Firearms Act regulated the information handgun shooting clubs had to provide police.
But the act appears to make no mention that clubs must have the address of the member stored in a database, only that they inform police of the member's name, and the firearms they used during competition.
Gun thieves have repeatedly been found to have stolen to order and supply the underworld, but police have previously denied that the thieves used information stolen from police or gun club databases.
This is despite numerous examples, particularly on farms and other remote locations, of properties with guns being ransacked while neighbouring properties that do not have registered firearms are left untouched.
The breach came to light amid internal ructions at the club following a decision by the Victoria Police licensing and regulation division to close its 25-metre shooting range on November 8 because of safety fears.
A person was injured by ricochet at the range and the club was found to have ignored recommendations made by the division in 2014, according to a notice sent by police to the club.
Two days later, club members started to receive unsolicited emails from an anonymous sender detailing various issues regarding the committee.
Club secretary John Scott wrote in an email to members that it did not appear anything other than their email addresses had been compromised, but that police would confirm the extent of the breach.
He fears the database was accessed improperly by people within the club who were opposed to the committee, and suggests that access to the database had not been properly restricted prior to the suspected hack. Personal information was improperly stored, he claimed.
"The incoming committee is taking these allegations very seriously and has already commenced measures to investigate," Mr Scott wrote.