The warning from the trade union Unison comes after Health Minister Robin Swann announced an increase in mileage rates for health workers – including home care workers and district nurses – employed directly by trusts.
Currently, staff are paid 56 pence per mile for the first 3,500 miles, and 20 pence for each mile thereafter.
The increase would bring the mileage up to 30 p per mile after the 3,500 threshold.
In a statement, a spokesperson for Unison said: “Unison members have heard today [Thursday] that mileage rates will be adjusted in response to their sustained pressure in the workplace and at the bargaining table.
“These allowances form part of the UK-wide terms and conditions and are currently under review.
“It is expected hard bargaining at a UK level will take place for an more sustained overhaul of the existing allowances.”
“Discussions are now under way to arrange the use of fuel cards and we are awaiting further information.
“There cannot be any delay in getting this support directly to staff who urgently need the means to keep their vehicles on the road while doing their job.
“What workers need is a decent pay rise.
“Unison across the UK is preparing for a sustained autumn campaign up to and including industrial action to achieve our goals.”