The Canberra Liberals will launch a sharp attack on the economic credentials of Chief Minister Andrew Barr, after an independent costing priced the opposition's payroll tax cuts well below Labor's early estimates.
But Labor maintains the policy would deter businesses growth in the ACT, because employers would face vastly increased tax bills when their wages went over $5 million a year.
The Liberals' promised payroll tax cut, which would only apply to businesses with national wages between $2 million and $5 million, would cost $48.1 million in foregone revenue over four years, ACT Treasury says.
ACT Labor had said the policy would cost about $500 million, assuming the policy introduced a new marginal tax rate, before the Liberals clarified the tax cut did not apply to wages paid above $5 million.
Opposition Leader Elizabeth Lee said a "simple fact check" would have saved Chief Minister Andrew Barr the embarrassment of releasing "outrageous figures".
"For Andrew Barr to put those figures out last week was desperate, disingenuous and Canberrans should expect better from someone who has controlled the budget for over a decade," Ms Lee said.
Mr Barr on Sunday said the Liberals had been gradually reducing the impact of the payroll tax policy.
"Tax brackets and tax policy are not necessarily the most riveting of election topics. But what this really highlights is the devil is in the detail and what they're announcing and then what they're submitting for costing are often two very different things," Mr Barr said.
Ms Lee also criticised Mr Barr's failure to submit a costing request for 800 new healthcare workers, a Labor commitment made in May.
"Theoretically, 800 new health workers could cost the territory over $200 million a year and Andrew Barr needs to tell Canberrans why he has failed to submit the policy to Treasury for costings before the deadline and how much it will cost," she said.
Mr Barr on Sunday said Labor had not submitted an individual costing request for the 800 staff, but the spending was factored into other costing requests.
"We didn't lodge it as a single policy costing. Individual initiatives within the 800 [healthcare workers] have been lodged, and are going through the costing process," he said.
"But the biggest single one, the 137 that kicks it off, was in the budget."
The budget allocated $86 million over four years to add 137 full-time equivalent nursing and midwifery positions within Canberra Health Services, with net new spending of $9.64 million over four years.
Debate over costings and policy detail is set to continue, with less than a week remaining before polls close in the 2024 election and almost 20 per cent of votes cast.
The Liberals said their payroll tax policy - a cut the payroll tax rate by 20 per cent for wages up to $5 million, with the rate to fall from 6.85 per cent to 5.45 per cent - had not changed, but the original statement did not make it clear how the policy would be implemented.
The Liberals requested a costing for their payroll tax policy last week, well after Labor had begun submitting requests for Treasury costings. The opposition had priced their payroll tax cuts at $80 million over four years.
Treasury said its costing "differs from that submitted due to the use of more recent data, and the use of unit record data for firms with national wages between $2.0 million and $5.0 million, based on the additional information sought in the clarification on the proposal".
The costing said 1007 firms with national wages between $2 million and $5 million paid ACT payroll tax in 2023-24; the average wages taxed per firm was $844,676. Treasury said it indexed payroll tax revenue growth using rates from the 2023-24 budget.
"This costing of the commitment would be sensitive to the growth in wages and number of firms in the ACT over time. There are firms with national wages under $2.0 million that do not claim the ACT tax free threshold; the proposed lower payroll tax rate was not applied to these firms in this costing," the costing said.
The Canberra Times estimated the Liberals' policy would cost the budget around $100 million over four years; the estimate relied on publicly available information.
Labor said at the time the Liberals' policy introduced a disincentive to business growth.
"The equivalent would be if once an individual's income went up into a new tax bracket the higher rate applied to all of their income," a party spokeswoman said.
"Not creating a new tax bracket reduces the cost, but creates a significant distortion in the payroll tax system where, for example, a payroll of $5,000,001 will attract a higher tax rate on the total amount above the $2 million threshold."
