MELBOURNE (REUTERS) - A Melbourne cafe went viral on social media this week after introducing an 18 per cent "man tax" to raise awareness of Australia's gender pay gap.

The gender surcharge at the Handsome Her cafe is optional and the proceeds are donated to a women's charity, cafe owner Alexandra O'Brien told the Seven Network.

"One week out of every month we have an 18 per cent premium for men, which is the same amount as the gender pay gap,"Ms O'Brien said.

The pay gap has ranged between 15 per cent and 19 per cent for the last two decades, the Australian Government Workplace Gender Equality Agency says on its website.

Ms O'Brien said the response from male customers had been positive.

"If people aren't comfortable with paying it, or men don't want to pay it, we're not going to kick them out the door," she said. "It's just a good opportunity to do some good."