The Mineral Resources Authority (MRA) of PNG has been advised and a full investigation into the incident has been launched.
Mining operations have been temporarily suspended and are expected to resume when it is safe to do so.
St Barbara is providing care and support to the family of the worker, the local community and all employees at Simberi Operations.
St Barbara managing director Craig Jetson described the news as "devastating".
"Our thoughts are with the employee's family, friends and colleagues at Simberi," he said.
"The safety and wellbeing of our employees is our highest priority. Safety Always is St Barbara's commitment to our people and the cause of the incident will be thoroughly investigated."
It is unclear if the incident will further impact Simberi's full-year guidance, which was downgraded earlier this week.
St Barbara lowered the full-year outlook for the mine from 95,000 ounces to 80,000-90,000oz and lifted all-in sustaining cost guidance from A$1720-1810 an ounce to $1790-2030/oz.
The operation has been impacted by ore variability due to low mining rates, as well as workforce availability in PNG.
COVID-19 flared up in PNG in late March, with the number of daily new cases rising over 550.
After dipping last month, over 400 new cases were reported per day for Tuesday and Wednesday this week, according to Johns Hopkins University.
St Barbara has had a number of Simberi employees test positive for the virus and two with pre-existing conditions died.
Group guidance for FY21 was lowered from 370,000-380,000oz at $1440-1520/oz to 330,000-360,000oz at $1547-1695/oz, with labour pressures also impacting Gwalia in Western Australia.