Tax officials have been told the agency blocked large numbers of calls from joining the queue to reduce wait times during an overwhelming spike in demand on Monday.

The surge hit the agency this week as tax officials answered questions late into the evenings about wage subsidies and other measures to absorb the economic shock of COVID-19.

Tax officials in the client account services branch were told record-breaking numbers of calls had been answered on Tuesday.

ATO staff are working at a relentless pace to answer calls, and hundreds have been redeployed from other tasks to try meeting the demand.

Tax officials have received requests to volunteer for overtime and prioritise answering calls if they've been trained, as agency leaders warn the pace won't slow soon.

ATO deputy commissioner Grant Brodie told staff in an email to the client account services branch there was "unprecedented" and "heavy" demand right until phone lines closed.

"You may not know this but we broke a record yesterday answering more calls in a day than ever before," he wrote.

"Amazing work, I realise for those of you taking those calls it's hard.

"One call to the next with no let-up in between, I realise it's tough and at times draining, but I wanted you to know that we so appreciate the massive effort you are putting in, trust me when I say it certainly does not go unnoticed.

"Thanks again for everything you are doing; we are without doubt making a difference in the community."

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Assistant commissioner Steven Atkins wrote to debt and lodgement staff on Wednesday, saying more than half its staff were directly taking calls, while the rest stayed on other "priority work".

Some staff who had begun working from home had quickly returned to ATO buildings to answer phone calls, he said.

Australian Services Union official Jeff Lapidos said ATO staff were working in a high pressure environment and doing their best to help employers and employees calling about the government's economic packages.

"They are finding they are dealing with a lot of distressed people, and those people share their anxieties and stress with the staff and that affects tax officials as well," Mr Lapidos said.

Many staff had been asked to answer calls at short notice and given inadequate time to train and prepare, he said.

"That's making it very difficult for them," Mr Lapidos said.

"To a large extent, people are learning on the job.

"While it's very difficult for people who have lost jobs, work hours and business, it's a difficult time for Tax Office staff who are trying to help them as well.

"It will help if people can be patient."

An ATO spokesperson said the agency this week had received high volumes of calls from people looking to apply to access their superannuation early, enrol for JobKeeper payments or to ask about the government's cash boost measure.

"We're doing our best to help as many people as possible," the spokesperson said.

"We ask that anyone having trouble getting through to us to please be patient and try again later in the day.

"Our staff have been working together to rebalance workloads and ensure additional staff are allocated to handle the increase in calls."

The Tax Office has no plans to draw on redeployed public servants from other agencies to help answer calls, instead relying on its own workforce and bringing forward start dates for staff usually brought in to support tax time.

"However, this position is being constantly evaluated to ensure the ATO remains in a position to deliver essential services, including programs recently announced by government," the spokesperson said.

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