Reports have emerged of businesses asking workers to only take part of the $1500-a-fortnight wage subsidy.

But Tax Office second commissioner Jeremy Hirschhorn said if the full amount was not passed on or if there was "some sort of kickback or commission" demanded by employers, the Tax Office would claw back the entire amount paid for all staff, and in the worst cases would refer the case to police.

"We ask any employee who has been approached, suggested, pressured into a kickback arrangement to approach us and we will be applying full resources to that behaviour," he said, speaking at a Senate inquiry into the government's coronavirus measures.

"We are hoping that the Prime Minister's very explicit statement to the nation is heard... It is only a few employers who would contemplate such a thing, but if we do come across them they will be treated very forcefully."

The $130 billion wage subsidy scheme has thrown up anomalies across the business world, with businesses complaining that some staff are receiving the payment but refusing to work, disputes between boss and worker about how many hours they must work, and workers complaining of unfair treatment.

The Tax Office has been inundated with calls, averaging 90,000 a day against the previous record of 71,000, and is redeploying hundreds of staff to the JobKeeper program. The inquiry head had received 1160 tip-offs through its hotline and about 2000 calls of concern about the program.

The government has a "one-in-all-in" rule where if a business claims the wage subsidy it must pass the $1500 to all eligible staff.

Mr Hirschhorn said the Tax Office had heard anecdotally about workers refusing to work or "shirking", and some saying they had been asked to work too many hours. Those complaints had been passed to Fair Work Australia.

In response to a question from Labor Senator Kristina Keneally about businesses making their own interpretations about which casual staff qualified, he said some were "grappling with more complex casual relationships particularly where people might have taken longer breaks in the last year". Those cases had also gone to Fair Work.

Cases where people had been asked to take annual leave and long service leave while they were on the JobKeeper payment were not relevant to the Tax Office, as long as the workers were being paid the $1500, he said.

Mr Hirschhorn reassured businesses that they would not be penalised if their turnover didn't fall by the required 30 per cent if they had acted in good faith.

"We ask people to make a reasonable estimate and a reasonable estimate is what is sufficient," he said.

"If it ultimately turns out that the estimate was overly pessimistic and a business only went down 29 per cent instead of 35 per cent that is OK ... where people make a good faith decision to comply and a good faith decision that they are eligible, the commissioner will be very understanding and sympathetic to their position, particularly where they have passed the benefit of the JobKeeper payment to their employees.

"I do hear this concern in the community that there will be some sort of nitpicking over-engineering of these tests ... what we are asking of business is to make a good faith effort and when we consider their good faith effort, even if it's a little bit wrong, we will be very sympathetic and understanding."

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Acting second commissioner Kirsten Fish said the Tax Office was applying flexible tests of turnover for businesses which existed but weren't operating in March last year, or those that had restructured in the past year, as well as those impacted by disaster or drought in the past year.

While 900,000 businesses initially expressed interest in JobKeeper, 768,000 had registered by Wednesday, officials said. While the government had expected six million workers to be covered, the figure stands at five million. Among big businesses with income of $250 million or more, 1378 had registered for the JobKeeper subsidy.

Treasury deputy secretary Jenny Wilkinson said numbers were "broadly in line" with what the department had expected, and she was not surprised more businesses had expressed interest than registered.

The department had done its best to estimate how many might register and how many workers might be eligible, but could not predict businesses adapting in the way they had, she said.

Businesses can continue to sign up until the end of May to receive the first payment, and beyond that can still join the six-month Job Keeper if their turnover falls.

So far, $2.4 billion in subsidies had been claimed, and $1 billion had been paid, with the first payments hitting bank accounts on Wednesday evening and Thursday morning.

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