By using blue zinc cream, the impression of the number is left on their skin. The stunt has been seen on at least two Sydney beaches.
One girl, Samantha, 15, told the Manly Daily: 'We come for both the beaches and the boys, but I'm taken so it's all good.
'I got a message and it said, "Your friends are hot - but what about me?"'.
However, her friend Lizzy, also 15, said their actions had also attracted the wrong kind of attention.
'An old guy tried to talk to me. He was trying to take pictures of us,' she said.
Campaigner Nikki Goldstein said: 'We'd never advise girls to broadcast their phone number on the internet or anywhere that could make them a target.
"They're in their bikinis, showing off their bodies, objectifying themselves in a way that is, to my mind, a bit trashy.'
Manly Police Commander Dave Darcy said: 'This is an issue where parents have to take up the ball and use this story as a discussion point with their kids.'
He advised youngsters to reflect on the implications of writing their contact details on their body.
'To me it is risky, you have no control over who gets your number. It is merely inviting trouble', he said. [via dailymail]