Hundreds of Apple Mac users including several in Australia, have seen their PCs plastered with porn after they downloaded what has now been identified as fake Mac software.Called MACDefender the software was downloaded from a web site which one victim described as looking “very legitimate”.

The program peppers screens with pornographic pictures shortly after being loaded.

Problems with the software were first identified after people affected logged onto forums to report their experience.

The BBC said that the fake Mac anti-virus software, which goes by the name of both MACDefender and Mac Security, began circulating in early May and has steadily racked up victims.

Such programs, often called scareware, urge people to install software that then pretends to scan a machine for security problems. It then fabricates a list of threats it has found and asks for cash before it will fix these non-existent problems.

Graham Cluley, senior technology consultant at Sophos, said the scareware’s creators had turned to search engines to get the program in front of potential victims by linking it with innocuous phrases such as “Mother’s Day”.

“You search for something on Google Images, and when you click on an image you are taken to a webpage which serves up the attack – regardless of whether you are running Mac OS X or Windows,” he said.

One trick the software uses to make people cough up cash quicker was to fire up the browser of unattended machines and call up one of several different pornographic websites.

Cluley said the vast majority of malware that Sophos and other security firms see is aimed at Windows users. About 100,000 novel malicious programs for Windows are detected every day, he said.