A Defence spokesperson told iTWire, in response to a query, that due to a potential security concern, "some elements of the Defence Force Recruiting Network were proactively taken offline on 2 February. Normal operation resumed on 12 February".
The ABC report said the database contained the details of "tens of thousands of ADF members" and it was "taken offline and quarantined from other military networks in February, while IT specialists worked to contain an apparent security breach".
The Defence spokesperson said an investigation did not identify any evidence that information had been compromised.
Contacted for comment, Lizzie Allen, sales and marketing director for ManpowerGroup Australia, said: "ManpowerGroup Australia is aware of a potential issue identified with the Defence Force Recruiting Network, requiring Defence to proactively take elements of this network offline.
"All elements of the DFRN have since been restored to full operations. All inquiries should be directed to Defence."
The ABC cited a source "familiar with the investigation" as saying that suspicions about the security of the database had arisen shortly before Christmas.
It also quoted Liberal MP Andrew Hastie, the head of the Parliamentary Joint Committee on Intelligence and Security, as describing the incident as a "very concerning development".
In February last year, it was revealed that the networks at the Federal Parliament had been infiltrated. A report later said Australian intelligence officials had concluded China was behind the breach, but had kept it quiet to avoid any problems in the bilateral trade relationship.
In June last year, the Australian National University suffered a massive data breach which led to the exposure of personal details of staff, students and visitors over the previous 19 years.
In October, the ANU released a report about the attack, giving a detailed timeline but no attribution for the hack or a possible reason as to why it was undertaken.
The ANU also had its cyber defences breached in July 2018.
Contacted for comment, a spokesperson for the Chinese embassy in Canberra said accusations against Beijing had been flung around on various occasions. "We challenge the so-called Defence insider or intelligence sources to come up with further evidence," the spokesperson added.