The female staff member allegedly had a knife pulled on her while walking back to her car in the hospital's public and staff car park around 6.45pm on Tuesday.
Senior Sergeant Jason Mccoy of Waitaki police said the woman was grabbed by a 23-year-old man.
"As she unlocked her car, this gentleman asked for her keys and pulled out a knife," interim Oamaru Hospital co-chief executive Andrea Cairns said.
"It was very traumatising for the staff member, who fortunately ran inside, back into the hospital's emergency department. The man couldn't manage to start her car, so he took off on foot."
Mccoy said the man then went on foot to an adjacent car park.
"He has then stolen another woman's vehicle and driven off," he said.
The man was "stopped shortly after on Thames Street by a police officer and taken into custody without incident".
Fortunately, neither woman was injured, Mccoy said.
The 23-year-old man had no fixed address in the South Island, he said.
He had come down from the North Island in the last two weeks, Sen Sgt McCoy said.
An email had gone out to Oamaru Hospital staff informing them of the event and offering support, Cairns said.
"I did come into ED and spoke to all the staff members that were involved, including the one that had been held at knife-point," she said.
"She is, under the circumstances, doing remarkably well. As you can imagine, it's quite a traumatising event.
"All the staff are quite shaken by it - it could've been potentially so much worse, we're just so thankful that it wasn't."
There had not been any problems with staff safety prior when heading to their cars before, Cairns said.
"We do have internal security measures in place, duress alarms for staff that they wear around their necks that alert police if there are any incidents within the building.
"Unfortunately, in this case, the staff member was leaving the building."
The car park was well-lit and staff were encouraged to walk to their cars in groups at night, she said.
"At that time of the evening, I guess you wouldn't be expecting something like this to happen."
The staffer's car was parked in a darker spot within the car park and the hospital had spoken with Waitaki District Council on Wednesday morning about additional safety measures, such as further lighting options, Cairns said.
The council was the hospital's primary shareholder and the owner of the car park.
Mccoy said the 23-year-old man was due to appear in Timaru District Court on charges of aggravated robbery and driving while forbidden.
"Police would like to thank the witnesses and members of public that came to the victims aid," he said.
"There is no place for this activity in our community."
