The Department of Health has called on Secretary of State James Brokenshire to clarify how much funding it will receive in the budget passed by Westminster.
In the absence of a deal to restore Stormont, the bill to put in place financial provisions for Northern Ireland was backed on Monday by MPs in the House of Commons.
It will go before the House of Lords on Tuesday evening.
The budget revealed an increase in health spending of 5.4% - but the union Unison has argued it amounts to a funding drop as it fails to keep up with inflation which stands at 6%.
Mr Brokenshire told Parliament he will also release £50m funding for health and education, as part of the £1bn deal agreed between the Conservatives and the DUP.
In a statement, a spokesman for the Department of Health said: "It has to be emphasised that health is facing very significant financial pressures.
"The initial assessment of the financial position for 2018-19 and 2019-20 indicates pressures of some hundreds of millions just to maintain existing services.
"In relation to the secretary of state's comments on funding from the confidence and supply agreement, immediate health priorities would include investment in waiting list initiatives.
Northern Ireland has been without an Executive at Stormont for 10 months, amid a power-sharing deadlock between the DUP and Sinn Féin.
Despite numerous round of negotiations since the institutions collapsed in January, the parties have not been able to reach a deal to return to government.
The main areas of disagreement relate to language and culture issues including the implementation of an Irish language act.
Introducing the budget at Westminster, Mr Brokenshire said he was reluctant to take the step but that Northern Ireland would otherwise run out of money by the end of this month without local politicians in place at Stormont to pass it there.
He added that he remains hopeful a deal can be reached between the parties, allowing an Executive and Assembly to still be formed.