SCC Tasmania released a statement on Monday outlining the dollar figure as part of an analysis of its payroll and rostering data being undertaken by KPMG.

The company expects the review to be completed in early January, after which repayments will start but it could not say how many people had been affected and in what circumstances the underpayments occurred.

SCC Tasmania chief executive officer Robyn Boyd said she "became aware" of the "potential underpayment" after taking on the CEO position in early 2020, and commissioned KPMG to undertake a review.

She said the underpayments were caused by outdated systems.

"It became obvious that there were underpayments primarily caused by outdated software versions of payroll and rostering systems, so ICT," Ms Boyd said.

She said the overtime was being recorded on the company's system, but discrepancies had occurred, which she blamed on a "complex" enterprise agreement.

"What happens, the sheer nature of them, is each time you rewrite or go past a nominal period for an EA and you go out to vote, it's added to. So you don't take away from it, you keep adding to it," Ms Boyd said.

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"And so that document becomes so complex and convoluted, so the interactions between the clauses and trying to keep abreast of them, the interpretation, and making sure that everything is as it should be, that really is the struggle.

"And then making sure that your technology is keeping up with those changes, and changes to the legislation too, that you ensure you are paying people correctly."

There is a statutory requirement that such underpayment investigations only need to go back six years.

SSC Tasmania will have investigated the matter for two years by the time it finalises its report, which it said was due to a large amount of documentation. Ms Boyd said SCC Tasmania had "the reserves" available to fully repay staff.

Health and Community Service Union Tasmania assistant secretary Robbie Moore said the union was concerned that the length of the investigation was bringing the six-year period progressively forward, potentially reducing the repayment amount.

He said staff still had no knowledge of the detail of how the underpayments occurred and who was affected.

"We're very disappointed that Southern Cross Care are not providing staff with more specific information, so we're undertaking our own investigations," Mr Moore said.

He disputed SCC Tasmania's claim that the complexity of its enterprise agreement was a factor in the underpayment.

"This is the largest aged care provider in Tasmania. You would think that when they go into the details of how to save money - like they do - they would pick up when they're underpaying staff, in the same way that they pick up things when they go against them," Mr Moore said.

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"It's probably less complex than most given their size. They're only having to deal with three different agreements. That in itself is not complex."

Even when the KPMG report is finalised, HACSU Tasmania says the underpayment matter is likely to end up in Fair Work to go over any discrepancies.

SSC Tasmania and HACSU also remain in negotiations over a new enterprise agreement, with a provision for unpaid meal breaks remaining a sticking point.

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Ms Boyd said they were pushing for unpaid meal breaks due to the company not wanting staff "to work through an eight-hour shift without a break", but Mr Moore said paid meal breaks are "vital" in the aged care industry.

HACSU was considering protected industrial action over the matter.

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