The websites of Kellogg's, Nutri-Grain, Vogel's and Specialty Cereals in Australia were hit in a string of mass defacements on Sunday.
The Anzac Day attacks were conducted by a single hacker, or hacking group, and affected Windows 2003 operating systems.
They preceded a spate of more than 70 similar attacks that hit the websites of Australian businesses and individuals yesterday, the bulk originating from a single hacking entity.
The Western Australian union initiative No Work Choices was defaced in yesterday's attacks, which replaced home pages with text containing an expletive.
Web defacements are a common occurrence on the internet and often use SQL injection attacks. They are considered by many to be a low-level threat.
The attacks follow a series of nine web defacements of sites owned by the Western Australian government, including the Government House and the City of Perth earlier this month.
The Federal Government was the subject of Distributed Denial of Service (DDoS) attacks and web defacements earlier this year by online activist group Anonymous. The group launched a coordinated assault on the Australian Parliament House and the Department of Broadband, Communications and the Digital Economy we sites in protest to the Federal Government's internet content filtering scheme.
It followed a DDOS attack on Australian Prime Minister Kevin Rudd's website late last year.
Members in the Labor caucus were quick to write-off the attacks as low-hanging fruit. Security experts say there are few effectual counter-measures against a DDoS attack.