An independent financial review of Wales' biggest health board has been commissioned by the Welsh Government.
It has also called the waiting time performance of Betsi Cadwaladr - under Welsh Government supervision for more than two years - "unacceptable."
Ministers have revised down the health board's deficit forecast from more than £50m to £36m for this financial year.
The Tories said the board's finances were "out of control" and it was one of Labour ministers' "worst scandals".
A report to the board had raised the prospect of the overspend growing to more than £50m without significant intervention.
The reasons for the overspend include the cost of agency nurses and emergency care.
Betsi Cadwaladr health board overspent by £20m in 2015-16 and £30m in 2016-17.
Three other health boards, including Cardiff and Vale, Abertawe Bro Morgannwg and Hywel Dda, are also facing financial deficits in the financial year 2017-18, amounting to more than £150m.
In a written response to Conservative AM Angela Burns, Health Secretary Vaughan Gething said all health boards had been set targets called control totals.
He said: "I have confidence that three of the organisations are currently on track to deliver these control totals, but I am disappointed that Betsi Cadwaladr University Health Board is not yet demonstrating the necessary action to deliver their control total.
"I will be considering the review's findings when the work is completed, and will seek assurances from the board that it is putting in place urgent actions to address the recommendations."
In a separate statement, a Welsh Government spokesman said: "We have made it clear to the health board that the current finance position and performance on waiting times is unacceptable."
Responding, Ms Burns AM, Welsh Conservative Shadow Health Secretary, said:
"That Betsi Cadwaladr finances are still out of control after two-and-a-half years of direct Welsh Government control is among the worst scandals to engulf this Labour administration.
"It's baffling as to why a financial audit was not instigated when the board first entered into special measures. Thirty months later, the Cabinet Secretary has essentially now ordered a review into his own department, as the buck stops with him where the performance of Betsi is concerned.
"Steps to address the degenerate situation are well overdue and if the downward spiral is allowed to continue then it's difficult to envisage a time when the health board will ever come out of special measures.
"For the sake of patients and hardworking frontline NHS staff, I sincerely hope the cabinet secretary will ensure that swift corrective measures are taken."
There was no comment from Betsi Cadwaladr health board.