NSW Ambulance paramedics attended Chinamans Beach in Mosman about 2.45pm today.
The woman, in her 30s, was bitten on the stomach.
The blue-ringed octopus is prevalent in rocky parts of Sydney beaches and along the east coast.
They range from the size of a finger to the size of a fist, with species in northern Western Australia tending to be bigger.
Read Related Also: Police storm into bodybuilder Yakiboy's gym and arrest him over a wild brawl on a busy Sydney street
Normally they do not bite unless they are threatened or taken out of their environment.
The blue colour only becomes easily visible when they are threatened, most of the time they look like regular brown octopuses.
Despite being very deadly and very common, deaths from blue-rings are very rare.
Sign up here to receive our daily newsletters and breaking news alerts, sent straight to your inbox.
