"JELLYFISH ALERT! The weird pose I'm doing is that of someone who has just leapt out of the ocean onto the handrail to avoid a pretty big jelly that had very long, suspicious looking tentacles," she wrote alongside the photo.
"I moved very, VERY fast indeed. P.S. Anyone know if this one is actually dangerous??? (I wasn't about to find out!) #Straya #TakinOnTheJellies."
One fan commented on the photo: "Yes anarexia [sic] is dangerous."
Refusing to take the comments lying down, Erin responded: "It is a terrible disease and can be dangerous to those afflicted, yes.
"I don't see how your comment pertains to a jellyfish? I'm sure they don't intentionally limit their food intake to stay thin?"
Despite the negative comments, many of Erin's fans showed their support for the star on Instagram.
"You are perfect in fact are what most women envy!," wrote one.
Another shared: "Your body is awesome and like you said it's fed your beautiful babies. Keep being an inspiration to many young woman."
"Some people are naturally slim, she's definitely not anorexic, she's lean and a healthy Mumma. Don't be so nasty," said another.