A cut to the Australian Taxation Office's headcount reaching 4600 in four years has left remaining staff struggling and the agency under-resourced, Labor says.
Agency annual reports show the magnitude of its recent downsizing under Coalition efforts to reduce the number of public servants.
Labor's acting shadow Assistant Treasurer Matt Thistlethwaite said the cuts had made life for the office's remaining staff harder.
"ATO staff are trying as hard as they can, but they've been left struggling thanks to these cuts by the Coalition, job cuts that have coincided not only with multiple web outages, but also billions of dollars in consultancy fees," he said.
"Labor believes in a strong, sufficiently resourced tax system that can work with parliament on important reforms like closing down loopholes for dodgy multinationals."
The ATO cut its headcount of staff from 25,093 at the end of the 2012-13 financial year to 20,435 last June as part of the Coalition's push to downsize the public service.
An ATO spokeswoman said it managed cuts to prevent any impact on revenue collections or service delivery, and had maintained balanced workloads for staff.
"We achieved this by making judgements about what is priority work, how we get the work done, and what work we could stop or reduce to maintain delivery of our commitments to government and the community," she said.
"We also reshaped and rebalanced our workforce, to ensure we have the right people with a combination of the right capabilities for now and for the next generation of tax and compliance experts."
But the Australian Services Union representing tax officers said the reduction left questions whether the ATO had the resources to enact an ambitious agenda set out by its boss, Chris Jordan.
ASU official Jeff Lapidos said while the agency recently had money needed to employ more staff, caps on staffing levels had stopped it from growing.
The office had a difficult few years since cuts in 2013 and following years, he said.
The ATO said staffing cuts played no role in problems with its IT systems.
"The nature of the unplanned outages experienced is varied and there is no single systemic or underlying cause of these issues," its spokeswoman said.
A series of IT outages has hit the ATO this year, stopping businesses and individuals from lodging their tax returns online.
Major ATO meltdowns in December, 2016 and July this year prompted widespread criticism and an admission from Tax Commissioner Chris Jordan that the ATO's reputation was being damaged.
In February, the ATO's website, online portals, Australian Business Register and business reporting channels were unavailable for two business days, while a series of outages hit users on April 19 and four separate dates in May.
