Insport has 17 stores in Sydney and is one of the larger companies in its sector.
The fact that data from Instore has been put online indicates that the company has not responded to the initial ransom demand from the attackers.
When REvil is used to attack a Windows system, the malware first exfiltrates the data to a site controlled by the attackers. After that, the data is encrypted and a ransom note generated.
A screenshot of the data leaked by the REvil gang. Names have been redacted.
If the victim refuses to pay up, the attackers publish the exfiltrated data online as a means of putting pressure on the victim to pay up.
Among the Instore data put online are customer lists, accounts statements, bank statements and employees' leave entitlements.
REvil is also known as Sodinokibi and was first spotted in April last year.
iTWire has contacted Instore for comment.
Ransomware researcher Brett Callow, who works for the New Zealand-headquartered security firm Emsisoft, told iTWire that ransomware incidents had morphed into data breaches and represented a risk not only to the target company, but also to its customers and business partners.
"Too many incidents are attributable to basic security shortcomings such as failing to patch in a timely manner," he said. "Companies must do more to protect both their own data and the data with which they are entrusted."