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Detective Senior Sergeant Richard Quested said about 5.20am a man entered the NPD petrol station on Evans St and demanded money.
"He was wearing a purple cap, goggles, an orange high-visibility vest and a dark long sleeved jersey with white print on the arm," he said.
"The offender left on foot with a sum of cash."
The victim was not injured during the incident, but they were shaken and are receiving support.
Anyone with information that may assist police in their investigation is urged to call 105 or ask for Detective Scott Genet at the Timaru Police Station.
Information can also be provided anonymously via Crimestoppers - 0800 555 111.