A doubling in call volumes compared to last year's tax time is forcing the Australian Taxation Office to draw on nearly 600 of its public servants to cope with the demand.

The ATO will train staff in answering customers' questions as the number of calls to the agency since July 1 passes 740,000, after Parliament passed the Coalition's promised tax boon for low- and middle-income earners reaching $1080.

The Community and Public Sector Union's deputy national president Brooke Muscat-Bentley says ATO staff told to back up the call centre fear returning to piles of unfinished work.

"Our members have great regard for their colleagues in the call centre, which is a stressful and difficult place to work at the best of times and the part of the ATO that has faced the worst of outsourcing," she said.

"However, this means their existing work is falling on remaining colleagues when they move to the call centre, or not getting done at all."

Ms Muscat-Bentley said the main public sector union's members in the ATO reported having little option to decline their bosses telling them to swell the ranks of staff answering calls.

One public servant described being "voluntold" to head to the call centre, she said.

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Some staff had to travel to new sites, often at greater distances from their homes than usual, throwing their childcare and other personal arrangements into disarray.

"We've seen members from teams across the whole ATO moved to take calls from taxpayers, including recent graduates, those at higher pay grades and doing higher duties, those in the small business team," Ms Muscat-Bentley said.

"It's still not enough people. Even with outsourcing and casuals, the ATO is struggling with call demand."

Budget cuts to the ATO and the Coalition's cap on public sector staffing had denied the agency "wriggle room" in meeting the influx in calls, she said.

The ATO, where staff also worked over the weekend in preparation for tax time, confirmed the majority of those backing up its call centre were redirected there. A "large number" had also volunteered, a spokeswoman said.

It was drawing on a large pool of staff to spread the load in meeting the surge in calls and reduce any back-up in public servants' regular work, she said.

She denied the diversion had caused personal inconvenience for staff.

"Their working hours are not changed or the office they are based out of," she said.

"They understand the need to be flexible and support the community in times of high demand and are generally happy to do what supports the community."

Payroll system changes and questions about accessing income statements on the MyGov website have also driven the surge in calls.