The child was also airlifted to hospital after being bitten on the leg while swimming in the same Whitsundays area in northeastern Australia.
A hospital spokeswoman confirmed to AAP the woman remains in a critical condition on Thursday morning.
She was treated for 20 minutes on the beach, having sustained "fairly large wounds with artery and bones, vessels, (and a) large haemorrhage" according to Queensland Ambulance Service's Tracey Eastwick.
The older of the women, keen snorkeler Justine Barwick from Tasmania, was swimming near a yacht Wednesday night when she was set on by the shark.
The most recent attack occurred late Thursday when a 12-year-old girl was bitten on the leg.
"Given how serious her injuries were and how far from shore and from medical help they were, without the rescue helicopter I don't think she would have made it back to the mainland alive", Mr McCauley said.
The girl, who was holidaying with her father and sister, is in critical condition and will have surgery.
Ms Eastwick said: "It is horrific ... for us as a community of paramedics it is quite confronting to have two similar incidents in the space of less than 24 hours".
"In north Queensland, shark attacks are not that common".
As the RACQ CQ Rescue crew were unable to winch off the yacht, the injured woman was put into a dinghy and taken 100 metres from the boat.
Doctor John Hadok, an off-duty emergency department doctor from Mackay Base Hospital, was able to attend the woman when she was pulled from the water, bleeding heavily from a wound on her leg.
"She really was incredibly lucky".
She arrived at the Mackay hospital about 8pm on Wednesday after the helicopter stopped at Proserpine to pick up blood and to refuel.