The 46-year-old suffered significant leg and torso injuries after being bitten at Cid Harbour, off Whitsunday Island, about 5pm on Wednesday.
She is being transported to hospital.
The last shark attack in the area was eight years ago.
Ms Barwick, a mother of two, would likely have bled to death without the quick-thinking actions of people in nearby boats.
She was pulled from the water and was bleeding profusely.
Experts say shark attacks are increasing as water sports become more popular and bait fish move closer to shore, but fatalities remain rare.
The Queensland government has said three drum lines will be installed in the Cid Harbour area on Friday to prevent further attacks.
Fisheries officers and water police are also patrolling the area, while swimmers were told to stay out of the water at the popular holiday hotspot.
The girl is in a critical condition at Mackay Base Hospital.
The RACQ CQ Rescue helicopter is circling overhead at the moment.
"She was very scared, cold and in shock - in and out of consciousness".
Less than 24 hours later, a 12-year-old girl was attacked in a similar area.
Emergency department doctor John Hadok.
Justine was on leave with her husband and friends enjoying time on their family yacht as part of an annual trip to the Whitsundays.
'Justine is a fighter, a fit, resilient woman who is an outstanding leader in the aged and disability care sectors, ' he said.
She arrived at the Mackay hospital about 8pm on Wednesday after the helicopter stopped at Proserpine to pick up blood and to refuel.