The group would use the money in different ways, including on holidays and paying off debts, a syndicate leader said.
The Queensland workers will each pocket A$2.6m from the winning ticket, lottery owner Tatts Group said.
The syndicate members all worked at the state's department of education, according to multiple local reports.
The winners, who wish to remain anonymous, used an office computer to confirm their windfall, Tatts said.
The Brisbane Times reported the 21 were part of departmental team of 35 people.
"We were talking in the office yesterday about what we would do if we won and now it has happened. I just can't believe it," the syndicate leader said in a statement.
"Now I'll be able to buy a house and get my teeth fixed. I'll be able to afford braces."
Some of her colleagues who also won initially thought it was a joke, she added.
The winners were aged between 23 and 60.
A department spokesman would not confirm to the BBC whether they employed the winners, but joked: "I can confirm it wasn't me".