Police charged the woman with armed robbery.
The incident happened at a department store in Kingsway shopping centre about 2.30pm.
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Police Media spokeswoman Susan Usher said staff allegedly saw the woman put items in her handbag.
“It will be alleged when staff asked to check her bag, she threatened them with a knife before walking from the store,” she said.
“A short time later, the woman was confronted by a security guard who also asked to look inside her bag.
“It is further alleged she threatened him with a knife, but he was able to restrain her until police arrived.”
Police charged the Greenwood woman with armed robbery, going armed to cause fear, drug possession and possession of stolen property.
Officers wish to speak with anyone who saw the incident at the store or with the security guard.