Eight cars were destroyed or badly damaged in a suspicious fire at Muswellbrook Hospital early on Sunday morning.
A hospital building was under threat from the fire, as the vehicles were parked in a row alongside it.
Fire crews managed to prevent the fire spreading by extinguishing the cars closest to the building first.
Fire and Rescue NSW's Muswellbrook station responded to reports of a car fire at 1.25am.
When the crew realised eight cars were on fire, further resources were called.
Edinglassie Rural Fire Brigade and Hunter Valley Police arrived soon after.
A Fire and Rescue NSW statement said the fire was being treated as suspicious.
Anyone with information can contact Crime Stoppers on 1800 333 000 or Muswellbrook Police Station on 6542 6999.