A humble Melbourne cafe that asks its male customers to pay an 18 per cent "gender surcharge" has found itself in the eye of a social media storm.

Handsome Her in Melbourne's inner suburb of Brunswick markets itself as a cafe "by women, for women".

As part of its three "House Rules", the cafe stipulates women have priority seating, men will be charged an 18 per cent premium "to reflect the gender pay gap [2016] which is donated to a women's service".

The third rule states "respect goes both ways".

Pictures of the sign were shared on social media where the cafe's business concept was debated — is this clever corporate social responsibility, or male discrimination?

To be clear, the surcharge is optional and only applies one in every four weeks.

What's more, the money raised by the surcharge is donated to Elizabeth Morgan House, Victoria's peak body for Aboriginal women's services.

The 18 per cent surcharge figure comes from a 2016 report from Australia's Workplace Gender Equality Agency that placed the average full-time base salary pay gap at 17.7 per cent, resulting in a difference in salary of up to $27,000.

The report also found that the more women there were in executive leadership roles, the lower the gender pay gap was in the organisation.

"Organisations with the lowest share of female executive leaders have an average gender pay gap double the size of those with an equal share of women in senior roles: 20 per cent compared with 10 per cent," the report said.

"Organisation-wide reductions in the gender pay gap were recorded for those companies that improved gender balance at the executive leadership level between 2015 and 2016."

The report included other findings such as:

Male graduates access higher pay, with men more likely to nap the top graduate trainee salaries.

Male graduates access higher pay, with men more likely to nap the top graduate trainee salaries. Women were much more likely to work part-time than men, and even out-earn men by about $4,000 a year. However, part-time managerial jobs tend to favour men, who earn more than 27 per cent than their female peers.

Women were much more likely to work part-time than men, and even out-earn men by about $4,000 a year. However, part-time managerial jobs tend to favour men, who earn more than 27 per cent than their female peers. The gender pay gap grows with seniority, resulting in an annual difference of more than $93,000.

But the case for raising money for charity and getting people talking about the "long forgotten gender pay gap" did not win everyone over.

"Whilst appreciate highlighting the issue of pay, creating an us and them is divisive. Flip this, and Twitter is in flames," one Twitter user wrote.

"I think if you want to fight for equality then surely treating everyone the same is the way to go," wrote Leroy Brown.

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Others supported the move, saying it was a fair move in considering that women are paid about 18 per cent less than men in certain roles.

"Love it! Spaces for ladies is always great and highlighting the pay gap like this is harder for people to ignore," Fiona Cannon wrote.

"As a man — no problem with it at all. No reason women shouldn't have their own spaces and be able to dictate what the rules are in them," Josh Elliott said.

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Meanwhile, staff of the cafe took to its Facebook page to address what they called a "hectic couple of days".

"In three days we have opened the cafe, withstood a social media storm … and gotten Australia talking about the long forgotten gender pay gap," the post said, thanking customers who had shown their support.

"I had a woman bring her daughters in today and when she came up to the till and saw our gorgeous vulva stones and our period sticker packs she beamed, thanked us for what we were doing and said 'what a beautiful place to take my daughters' … I swelled with pride," the Facebook post said.

"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."