A new report by the Australia Institute's Centre for Future Work found the average employee worked 6.13 unpaid hours each week in 2021, or eight standard working weeks per year.
That's compared to 5.25 hours per week in 2020 and 4.62 hours during 2019.
The institute's report, released to coincide with Go Home on Time Day, found $125 billion was lost in income from Australian workers during the past year alone.
Centre for Future Work economist Dan Nahum said the pandemic and related lockdowns were making the situation for unpaid work worse.
"In many cases, it's making it easier for employers to undercut Australian minimum standards around hours, overtime and penalty rates," he said.
"We're seeing further incursion of work into people's personal time and their privacy."
Workers aged between 18 and 29 performed the most unpaid overtime in the past year, at an average of 8.17 hours per week.
Part-time employees were working for free for 4.5 hours per week while casuals were doing unpaid overtime for more than five hours every week.
During the pandemic, more than one-quarter of workers surveyed reported the employers' expectations of work availability increased.
"This year, Australian workers are taking home a smaller share of GDP than we have ever seen before," Mr Nahum said.
"Yet time-theft is rife and bosses are stealing record amounts of unpaid time from workers."
Almost 40 per cent of employees reported their workplace was monitoring them through technology such as web cameras or keystroke counters while at home.
With more people working from home following COVID-19, the report said the nature of employees working unpaid hours had also changed.
"Now we must consider whether home work will become the new normal for many workers, even after the acute phase of the pandemic finally passes," the report found.
"Whether working from home or at a formal workplace, the problem of unpaid work continues to be severe."