An armed man brandishing a machete and wearing a bandana robbed a South Island dairy on Christmas night.

The aggravated robbery happened just before 10.30pm yesterday at the Night and Day dairy in Stoke.

Police said the offender entered the dairy and demanded money from the lone female employee.

He is described as Maori or Polynesian, mid-teens to early 20s, slim build, and about 5ft 10 inches in height.

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The man was wearing a dark blue, sleeveless hoody, with a silver/grey coloured design on the back, a long sleeve black top, dark tracksuit style pants, and a green bandana covering his face.

"He was carrying a small dark coloured backpack bag with grey and yellow circle design on the front and was holding a black handled, short, machete-style knife with electrical tape wrapped around the handle.

"An amount of cash was taken during the robbery with the male last seen riding a bicycle through Strawbridge Square, then north along Neale Avenue, Stoke."

Police said the shop employee was shaken and upset but unharmed. She was being supported by friends, family and victim support.

Police ask anyone who saw someone matching this description in the Stoke area last night to contact Nelson Police or report anonymously via Crimestoppers on 0800 555 111.