Zoo Weekly has urged men to submit photographs of their girl's cleavage so that readers can vote online for which woman most deserves 10,000 dollars (8,450 US) worth of plastic surgery to improve her chest.
"It's impossible to think of a more romantic gift than new breasts," magazine editor Paul Merrill said in a statement.
"It's the gift that keeps on giving."
But health experts attacked the competition, describing it as unethical and in poor taste.
"I'm disgusted and appalled, and very doubtful they can even offer major body modification as a prize," said Jenny O'Dea, a public health researcher at Sydney University.
"You simply cannot treat women in this way, like objects there for men's satisfaction."
The Australian Society of Plastic Surgeons said it was medically unethical to offer surgery in a competition and that it was inappropriate for a man to win such a prize and offer it to his girlfriend.
"What would we think if a women's magazine ran a lottery for a penis enlargement and asked women to volunteer their boyfriends," society President Howard Webster said.
Sexuality researcher Julie Mooney-Somers said it was also possible that women could be entered into the competition without their knowledge.
"What's to say these women even want a boob job or that it's even safe for them to have it," said Mooney-Somers, an academic at the University of New South Wales.
"There may be some very horrified women out there thanks to this."