The company said in a statement on its website that the breach had been noticed on 23 May and five days later investigations showed that client data may have been compromised.
Chief executive and co-founder Karen Cariss said a third party was providing assistance in a forensic probe into the breach.
"...we can share that the source of the incident was a malware infection," Cariss said. "The malware has been eradicated from our systems and we have confirmed that our anti-malware signatures can now detect the malware. We see no further signs of malicious or unauthorised activity and are confident in this assessment."
Wonder what the odds are this would have been reported without mandatory data breach notification laws...— Shogun Cybersecurity (@ShogunCySec) 6 June 2018
She said that if personal data had been leaked, then it could include name, contact details, ID and authentication data such as usernames and passwords which had been encrypted.
FFS. PageUp has no business retaining TFNs at all. They can collect and pass off to employer but retention is balls out dumb. https://t.co/jjfuGSMaNa— Cail Young ? (@cailyoung) 6 June 2018
Australia Post said it was "managing an issue following advice from PageUp, a third-party supplier that has helped us process external job applications since October 2016, that they’ve experienced a system breach".
It added that it had shut down its careers website. Also shutting down jobs websites were Telstra and AGL.
Coles said it had suspended all connections between its systems and those of PageUp until it gauged the extent of the breach.
"We have asked for urgent responses from PageUp and are also conducting our own investigations," the supermarket said in a statement.
"Coles is not currently aware of any fraudulent activity relating to anyone's data occurring as a result of the security breach.
"However, we recommend that any person who has applied online for a position with Coles in the past 18 months check to ensure that there has been no recent unusual activity concerning their personal information and maintain a close watch on the use of their personal information."
Commenting on the breach, Tony Smales, chief executive of security outfit Forticode, said: "An unfortunate side-effect for them will be the loss of trust in their brand and service."
He said as PageUp had a global customer base, the company would come under both Australia's mandatory data breach law and also the General Data Protection Regulation in the EU "where the fines can be significant – especially if health, financial or government associated information is breached such as social security".
Smales said as people would have provided personal and identifying information (such as past workplaces, contact details for referees, birthplace and addresses) to PageUp, the breach would directly affect not just them, "but the entire industry which places trust in these services".
Additional implications were the fact that a complete profile could be built from the data that had leaked, he said. "A thought I often have in this age is when such a fraud event occurs, who will the insurance companies come after? This is on top of any legislative events or fines issued by government.
"If Company A loses your TFN and then a fraud event happens with the ATO, using that TFN, who should ultimately shoulder the accountability associated to that event? The company that asked you to put your information into Company A? Company A itself? The Insurer? You? Or the ATO? It’s a bit of a hornets' nest."
Smales added that it would be possible for those whose details had been leaked to be easily drawn into phishing scams.