Credit ratings agency Moody's has downgraded the debt rating for Tasmania, predicting widening budget deficits.

While the state recorded a $60 million surplus for the 2015-16 financial year, its rating has fallen from Aa1 to Aa2.

In a statement, Moody's disputed the Treasurer's spending projections in the May budget.

"Moody's anticipates that Tasmania will record widening deficits due to a combination of slowing revenue growth and expectations that the state's forecast of nearly flat levels of spending are unlikely to be realised," the agency said.

Moody's noted the State Government's figures rely on ''virtually no growth in healthcare costs over four years".

"Moreover, upward pressures on public sector wages could emerge, particularly as relevant agreements expire over the next year and the number of full-time employees is beginning to rise again," the agency said.

Treasurer Peter Gutwein was taken by surprise, since Moody's offered more positive feedback only last week.

"The adjustment highlights that there is still more to be done," he said.

"And while the budget is back in surplus four years ahead of time there remains significant long-term challenges in the areas of health, education and the state's superannuation liability that need to be managed carefully."

Moody's assessment harsh: Eslake

Independent economist Saul Eslake described the downgrade as "harsh" and added it was unexpected.

"While I, having read Moody's statement, understand why they've made the decision that they have, I think it's a harsh judgement, and one that may well turn out to have been based on invalid and incorrect assumptions," he told 936 ABC Hobart.

Mr Eslake said the Government had form when it came to maintaining budget discipline.

"It's certainly going to require concerted discipline by the State Government, especially in the lead-up to the next election, to keep spending at the levels foreshadowed in the most recent budget," he said.

"But they have at least shown that discipline over the last two years."

Government's credibility in tatters: Opposition

Shadow treasurer Scott Bacon said Mr Gutwein's economic credentials were in tatters.

"It highlights the fundamental, damning lie behind Mr Gutwein's budget - a budget which is propped up by a dividend from the TT-Line that he cannot spend and his not-to-be-believed growth targets," he said.

"They need to explain to Tasmanians why they have been arrogantly talking up their so-called credentials when things were going so horrifically wrong and they knew about it at the same time."