A Melbourne cafe went viral on social media this week after introducing an 18% "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% premium for men, which is the same amount as the gender pay gap," O'Brien said.

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

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

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."

On Facebook, the cafe said it has been "jam packed with people showing their support" since the story appeared - from both genders: "We've had men travel across town to visit us and pay 'the man tax' and throw some extra in the donation jar - guys, you're pretty neat."

"Who would have thought one little chalkboard would cause such a stir?"

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