"NZ Police is also offering assistance to Joanna's family in New Zealand."
Her sister Maria Ji told the Herald that Joanna, a former University of Auckland law student, has been in Amsterdam since December on a working holiday visa.
"Usually she would message my mother every other day, but suddenly she just stopped from last week," Maria said.
"Then her handbag, wallet and mobile phone was found by a member of the public and turned in to the police there, so we know she doesn't have any of those belongings or money on her."
Joanna Ji was last seen on Wednesday, April 20 in Amsterdam. Photo / Supplied
Maria said Joanna was an active user of social media and has not been in contact with any of her friends.
"We are very concerned because we don't have any family in Amsterdam and Joanna doesn't have many friends there that she could have stayed with," she said.
"We've filed a missing persons report with the police here, and they told us they are working with Interpol. Our family are planning on flying over there soon."
It is not known what Joanna was wearing at the time of her disappearance but she often wore a pink beanie, Maria said.
Joanna's hobbies include ice skating, music and fashion.
Maria described her sister as a creative, outgoing and bubbly person who is really empathetic and passionate about lots of social issues including human rights, animal rights and gender equality.
"Our concern is that she has been harmed or kidnapped, and we're really appealing for any information to know that she's okay," Maria said.
"Her former flatmate in Amsterdam has also filed a missing persons in the local station in Amsterdam and we've set up an email for people to get in touch."
Anyone with information can email: lookingforjoanna@gmail.com
