A child suffering from smoke inhalation had to wait for an ambulance to come from more than 30 miles away – despite living just 10 minutes from a hospital.
It happened as A&E units at both the Downe Hospital and Lagan Valley in Lisburn closed at the weekend for the first time.
The child, who lives near Downpatrick, had to wait for a paramedic travelling from Newtownards to be treated.
The young patient then faced a further delay for an ambulance to arrive from Newry, despite living 10 minutes from Downe Hospital.
A critical shortage of staff has led to the introduction of weekend closures, which have sparked fears that someone will die if the full emergency service is not restored.
Sinn Fein MLA Chris Hazzard said the incident involving the child was completely unacceptable, adding he was concerned for patients living in rural areas needing emergency treatment.
He said: "We are just very, very lucky that a couple of days after these changes were made we are not seeing far more severe consequences. I firmly believe that lives will be lost if the service is not restored at weekends."
He said he will be seeking an urgent meeting with Health Minister Edwin Poots about the issue.
The reduced hours, which came into effect on January 4, means the units at both hospitals are only open from Monday to Friday, 8am to 8pm.
The Health and Social Care Board (HSCB) said a review indicated the weekend passed "without major difficulty".
Additional nurses are now rostered at the Royal Victoria Hospital with six additional in-patient beds, as well as extra A&E nursing staff and senior doctors at weekends. The Ambulance Service also put in place extra crews in Downpatrick, Lisburn and Comber.
HSCB chief executive John Compton said the changes did not have any significant impact on pressures at other hospital sites.
"Staff ensured that a range of measures, including extra support for GP out-of-hours in the Lisburn and Downpatrick areas and direct admission to Lagan Valley and Downe Hospitals via the GP out-of-hours service, worked very well, ensuring patients from these areas continued to receive the care they required," he said.
But Seamus McGoran, director of hospital services at the South Eastern Trust, said it had struggled with staffing issues for years.
He said that after two A&E doctors left in November the situation deteriorated.
"For us to deliver two emergency departments at Lagan Valley and the Downe we would need eight middle grade staff at each of those sites – 16. We have six," he said, adding it was facing 70 unfilled shifts this month.
A Department of Health spokesman said: "The minister has asked the HSCB and the trust to make fresh efforts to secure medical staffing for both the Downe and Lagan Valley emergency departments. The board and South Eastern local commissioning group will be working with the South Eastern trust to accelerate plans for the long-term future for the Downe and Lagan Valley emergency departments."