The federal budget watchdog says public servants take an average of 11.5 paid sick days a year, compared with 18 days reported by Treasury Board President Tony Clement.
The parliamentary budget office says in a new report that Clement's number includes time missed due to workplace injuries and unpaid sick leave.
It estimates the salary paid for sick days amounted to $871 million in 2011-12, about 68 per cent higher than the estimate 10 years prior, which includes inflation.
But it says growth in the size of the public service, wages and number of paid sick days available all contributed to the doubling of sick-leave costs in the last decade and the use of sick leave between departments varies significantly.
The report comes as Clement prepares for a battle with public-sector unions in his quest for budget savings.
The governing Conservatives, who are tabling a budget next week, are taking aim at public service salaries, pensions and sick leave in their efforts to balance the books in 2015.