The attack was one of the largest, most systematic defacements of worldwide government servers on the Web, Attrition.org said on its site.
The sites were replaced with the name of the hackers' group, Pentaguard, and the message: "presents... the largest .gov and .mil mass defacement in the history of mankind."
While the defacement message posted was the same on all Web sites targeted, the attack was unusual in that this was not a mass defacement of a series of government Web sites within an agency or even within a country -- but of three different nations in different time zones all at the same time with all sites remaining defaced for a period longer than 15 minutes, Attrition.org said.
In Australia, 15 government sites were defaced. Some of the sites included those belonging to the Australian Institute of Marine Science, Ports Corp South Australia, Queensland State Archives and numerous local council sites.
Among the US sites defaced were those of the Republican Caucus for the California Legislature and the Alaskan Office for the Department of Interior, according to Attrition.org, which has mirrored the defaced sites on its servers.
The UK Web sites defaced included those of a number of local authorities, and a Web site set up by the UK government to disseminate information about the current epidemic of Bovine Spongiform Encephalopathy (BSE), otherwise known as Mad Cow Disease.
A number of US government and military Web sites, including those of the Army, Navy and Air Force, the National Institute of Health, the Department of Treasury, the US court system, the National Aeronautic and Space Administration (NASA) and the Department of Energy, have all previously fallen victim to hackers claiming to represent the same group.
The group has also previously defaced sites of the governments of China, Kuwait, Romania, Georgia and Vietnam, Attrition said.