LandMark White said in a statement on its website that it had shut down a security vulnerability on 23 January but was not aware at that point that data had leaked.
The company, which is one of the bigger property valuers in Australia, said on 5 February it became aware through corporate partner CoreLogic "that a dataset containing property valuation and some personal contact information had been disclosed".
LMW said the data that was at risk was "property valuation and some personal contact information of home owners, residents, and property agents, including first and last name, residential address, and contact numbers".
LMW said on 6 February, its last update on the breach, that it had hired unspecified leading external cyber security and privacy experts to undertake "a forensic investigation into the incident, as well as the circumstances of the disclosure of the dataset".
ANZ chief data officer Emma Gray said in a statement: “ANZ is aware of this industry-wide incident. We are currently undertaking investigations to understand specifically which ANZ customers may be affected and we will contact them directly to outline potential impacts and how we will support them.
“At this stage we understand a very small percentage of our customers who had valuations undertaken between November 2015 and December 2018 are potentially impacted.
“ANZ uses a range of property valuers and the organisation in question represents a very small portion of the valuations conducted. As a result of this incident ANZ has currently suspended use of the services of the valuation provider at the centre of the investigations.
"We have no reason to believe any of the other valuers ANZ uses are impacted by this incident. ANZ takes its privacy obligations very seriously and we are extremely disappointed this incident has occurred. We are now firmly focused on supporting our customers through this incident.”
Contacted for comment, a Commonwealth Bank spokesperson said: "LandMark White Limited made us aware of a data incident, which involved a number of financial institutions, including CBA Group. As part of the data incident, customer information relating to property valuations was found hidden on the Internet.
"CBA’s systems were not breached, so we can reassure our customers that no bank account information was disclosed as part of the incident. The customer information that was disclosed relates directly to the valuations completed by LMW and includes customer name; contact details such as phone or email address; and details about the valued property.
"CBA is contacting all affected customers to advise of the proactive steps the Bank has taken in response to this incident, which includes heightened monitoring of their accounts.
"We apologise to our customers. We take the protection of data and security incidents very seriously. The safety and security of our customers’ information is of paramount importance to us, which is why we have immediately suspended using LandMark White while we investigate how this occurred."
A NAB spokesperson told iTWire the company was aware of an issue "involving a possible breach of information from property valuations completed by independent valuation provider LMW".
"Some customers of NAB and other companies are potentially impacted. We are in the process of identifying NAB customers and taking the right steps to contact them directly," the spokesperson said.
"We take the safety and protection of our customers' information very seriously. Importantly, NAB's systems remain secure."
Commenting on the breach, Mark Perry, APAC chief technology officer at Ping Identity, said: "The LandMark White Limited data breach suggests that APIs are becoming an increasingly common attack vector and this has been proven in a range of API-driven businesses over the past 12 months, including at Facebook. API-based organisations are simply not secure these days using traditional cyber security solutions such as application firewalls.
“Security teams must now find a balance between making their APIs easy to use and having them tightly controlled to prevent misuse. They must ensure security is built in at the design stage and include features such as tokenisation, edge security, threat detection and machine-to-machine protection.
"Moreover, as APIs will continue to play an increasingly important role within enterprise IT infrastructures, IT departments shouldn’t view API security as a one-off exercise.
"Chief security officers need to constantly discover, monitor and secure the infrastructure. This, combined with the ongoing evolution of infrastructures from walled gardens into dispersed networks, means the importance of effective API security will remain critical in the years ahead.”