Ministers have not been clear in explaining why an extra £180m has been earmarked for the NHS, a committee of AMs has said.
But AMs warned there was no incentive for health boards to implement stronger controls if they are bailed out.
The Welsh Government said it had been "transparent".
The criticisms were made in a report published by the finance committee.
The extra £180m, included in the second supplementary budget for 2016/17, would be used to support Hywel Dda and Betsi Cadwaladr health boards which would otherwise not balance their books by the end of the financial year, with an extra £50m allocated for winter pressures.
Simon Thomas, chairman of the committee, said: "We want to see more detail in the Welsh Government's plans, including why two health boards need an extra £70m to make up the shortfalls - something the NHS Finances Act, which allowed health boards to plan over three years was designed to avoid."
Committee members asked why, given the strains health services are under every winter, the Welsh Government had not worked with health boards to ensure effective planning takes place.
"Should the Welsh Government continue to bail out health boards who overspend, there is no incentive for these health boards to implement stronger spending control to ensure they remain within budget," the report said.
AMs called for ministers to review whether there were structural factors that affected health boards' ability to deliver services within their resources.
A Welsh Government spokesman said: "We have been clear and transparent about why additional funding is needed in the health service and more information is readily available on our website.
"The extra £50m we put into the Welsh NHS to sustain performance and meet the increased demand into the winter is being used to continue the good progress made towards improving waiting times.
"This is benefitting patients across Wales.
"It is important to recognise that six out of 10 NHS organisations have approved medium-term plans in place.
"However, some organisations' financial planning regimes have been more successful than others - those who are struggling are receiving focused intervention from the Welsh Government."