The woman had no fractures but her face was badly swollen.
"She's not going back to work, not with her face looking the way it is.
"She's traumatised . . . she didn't know what had hit her because it all happened so fast."
The salon closed for the day as the workers were "in shock" from the incident.
The woman had never met her attacker but was told she allegedly had a connection to a relative some time ago.
"My heart goes out to my sister, I wouldn't wish this on anyone, especially in a workplace in public. It's horrible."
Police were called to the mall about 4.20pm on Thursday, a spokeswoman said.
An 18-year-old woman was arrested and charged with assault with intent to injure and is due to appear in Hutt Valley District Court next week.
A Wellington Free Ambulance spokeswoman confirmed they treated one person at the scene for minor injuries.