Police said they were called about 11pm after a man got into the back seat of the woman's car and pointed a gun at her, telling her to drive.
"The woman drove a short distance before getting out of the car and asking for help at a nearby house. The man fled on foot," they said.
"Fortunately, no one was injured, but this would have been a terrifying experience for the woman."
Police are investigating, including making forensic inquiries, to locate the man.
Health NZ said it had boosted security at the hospital.
"This has been a traumatic experience for our staff member, and we are wrapping all the care and support around them that we can," said MidCentral Health's group operations director Sarah Fenwick.
"We have put additional security measures in place and will continue to speak with our teams about what more we can do."
An internal notice to hospital staff, seen by RNZ, said security would be available to walk staff to their cars if they needed it, and all afternoon and night shift workers would have access to the staff car park.
They encouraged people to "walk with a buddy" before and after their shifts if possible, lock their cars, and keep valuables out of their cars.
