The ACT government's future budget estimates are "fanciful" and couldn't be achieved without cuts to services, a former treasury official says.
Former executive director of policy at ACT Treasury Khalid Ahmed said Andrew Barr's 2019-20 budget lacked credibility and sustainability.
He said it appeared unlikely the budget would return to surplus in 2021, as predicted by government, especially to the extent it has projected in forward estimates.
The June territory budget forecast that over four years, growth in government revenue would rapidly increase but there would be a sharp drop in spending growth.
The government has projected revenue growth will increase from 4 per cent in 2019-20 to 7 per cent in 2022-23. In that same period it has predicted spending growth will decrease from 6 per cent to 3 per cent.
Dr Ahmed said that accounted to a "miraculous growth in revenue". He said the government would need to show a lot of spending restraint to achieve the predicted slowdown.
"The figures are not credible, that growth in revenue is not sustainable," he said.
This year's budget predicted a surplus of $43 million before an $89 million deficit next financial year, but returning to surplus the year after.
But by 2022-23 the surplus is projected to jump to $400 million. Last year's budget had predicted a surplus of about $50 million in 2021-22. Dr Ahmed said the figures were unsustainable and overly optimistic.
With the projections forecasting the deficit to hit $100 million sometime next year, it would require a turnaround of half a billion dollars to reach the projected 2022 surplus.
"These forward estimates are quite heroic," Dr Ahmed said.
"Either forward estimates are fanciful or, yes, there will be real cuts to expenditure."
Dr Ahmed said if the government was to achieve a slowdown in spending it would impact services. He said health funding was likely to suffer from any cut in spending.
He said the budget gave health a low priority with an increase of just 3.9 per cent per year over the forward estimates. This did not meet inflation costs.
"Budget estimates provide no capacity to meet increased demand from population growth, ageing and new technologies," he said.
"Unless policy decisions are taken to allocate more funding, or curtail access, further pressures in the hospital system are certain."
An ACT government spokeswoman said the revenue growth projections in the budget were primarily driven by growth in the territory's population and economy.
"For example, ongoing growth in employment contributes to increased payroll tax revenue, and growth in the number of homes built for new residents grows the rateable property base," she said.
The government said expenditure increased each year in forward estimates, but did not address a question about the slowdown in growth.
Dr Ahmed said taxation had grown 6.8 per cent between 2012-13 to 2017-18, despite a much slower growth in the economy.
He said the government's concessions system had not kept pace with an increase in taxation, placing a disproportionate burden on low to moderate income households.
"If the government keeps increasing their tax revenue people on high incomes can manage, but the disadvantaged will suffer," he said.