ALDI has warned those who have used the system that any information they submitted could have been compromised.
Several companies have stopped using PageUp People's site altogether after the breach announcement, among them Monash University.
In an email sent to those who have used its online recruitment system, ALDI said on Tuesday that the information exposed may include names, addresses, phone numbers, work history and email addresses.
The company initially said the breach was due to a malware infection. But a statement on 12 June said, "Advanced methods were used to gain unauthorised access to PageUp’s IT systems in Australia, Singapore and the UK," indicating that whoever gained access had gone deeper into company systems than originally indicated.
"When we were informed of the incident we immediately suspended our use of PageUp while the matter was investigated," ALDI Australia's corporate finance and administration director, James Buonpane, said.
"Since this time we have been working with government bodies and privacy and information security experts to understand the impact of the incident."
Buonpane said while people who had used online recruitment systems at other companies which also used PageUp People's systems would send users emails similar to those sent by ALDI, the data that was affected could be different depending on the business involved.
In a related development, the Australian Cyber Security Centre, the Office of the Australian Information Commissioner and non-profit security provider IDCARE issued a joint statement on Monday, saying that there was a possibility that no Australian information had been exfiltrated.
The statement said that PageUp believed "that certain information pertaining to staff members, applicants and referees was accessed by an unauthorised third party".
IDCARE managing director David Lacey said the direct risk of identity theft was unlikely, adding that there were other risks likely to be more relevant to affected individuals, "including the possibility of phishing emails, telephone scam calls, and specific risks to individuals concerned about their contact information, physical address, and employment details (and applications) becoming known to third parties".
PageUp People's website is hosted by Amazon and appears to run on Microsoft's IIS Web server. There have been many cases where data was left unsecured in AWS buckets and leaked as a result, though this may not have happened in PageUp's case.
The HR services provider has among its Australian clients the Commonwealth Bank. the Australian Broadcasting Corporation, Telstra, NAB, Coles, Aldi, Medibank, Australia Post, Target, Reserve Bank of Australia, Officeworks, Kmart, Linfox, AMP, Asahi, Sony, Newcrest, the University of Tasmania and Lindt.