An union representing ambulance workers says scrapping paid meal breaks could create problems in the service.
Paramedics can volunteer to answer calls during breaks
Under a new working agreement, workers are considered off duty at meal times and do not have to answer 999 calls.
Unison said the system relies too much on paramedics who volunteer to work during their breaks.
The Welsh Ambulance Service said a new management system will monitor breaks and crews can take them or choose to be paid to work through break times.
Until the new agreement, paramedics in Wales were paid for time spent on breaks, but could be interrupted to answer emergencies.
In the new agreement between unions and the trust, staff are being given the option of uninterrupted breaks without pay or to make themselves available to be called out and receive a special payment as a result.
Unison has said the system relies too much on paramedics who volunteer to work during their breaks, but added it would not have signed up to the agreement if it did not believe enough safeguards had been put in place by the Welsh ambulance service.
Joe Conaghan, a paramedic who represents Wales on the Unison UK ambulance sector committee, said: "We don't know until the whole thing pans out how many of the crews will volunteer to be available.
"At this stage, we are convinced that we can make this change without any undue risk to the public.
"But we do have some concerns that this should not be a long-term fix and if it's an issue of funding then we need to persuade both the Welsh Assembly Government and the UK Government that this situation cannot be sustained for much longer."
Dave Galligan, Unison's head of health in Wales, said the new system needed to be closely monitored.
He said: "It is, effectively, hugely dependent of crew good will.
"The crews will be asked whether they're going to be available during their meal breaks.
"If they are, there's a notional payment and that will provide cover.
"But what you can't afford to have is a service that's shut for lunch."
Chief executive of the Welsh Ambulance Service NHS Trust, Alan Murray, said new shift rotas were being designed to increase cover at meal break times.
He added: "We won't be shut for lunch because we stagger shift start and finish times and therefore we stagger lunch times.
"We've got a window of three hours to get people their lunch and we'll be putting in meal break management systems to ensure we do that safely.
"We are boosting the cover over the meal period.
"The majority of ambulance trusts in the UK are doing exactly what we are doing perfectly safely."