Western governments are increasingly calling for Assange to be stopped as WikiLeaks continues to publish more than 250,000 confidential US diplomatic documents, the Sydney Morning Herald reported.
When asked what Australian laws have been broken by either WikiLeaks or Assange, Gillard said the Australian federal police were investigating.
"The foundation stone of it is an illegal act," Gillard told reporters Tuesday.
But the "foundation stone" was the leaking of the documents to the website, not the publishing of the cables, the report said.
"It would not happen, information would not be on WikiLeaks, if there had not been an illegal act undertaken," Gillard said.
It is widely assumed the man responsible for the leaks is a US soldier who is already in prison for previous leaks.
Attorney-General Robert McClelland said Monday that he believed the release of the cables could threaten the lives of people providing information to intelligence and law enforcement officials.
The federal police were not only looking at whether any Australian law had been breached by Assange, but would help US law enforcement authorities in their investigations, he said.