Grace Millane from Essex, England, was last seen in Auckland, last week Saturday.
Auckland City Police detective inspector Scott Beard said Millane had not made contact with her family on the day she went missing.
“This is particularly out of character, especially as it was her 22nd birthday on Sunday,” said Beard.
“We are working closely with her family at this time and a number of Police staff are currently working on this matter.”
She arrived in New Zealand in mid-November – and was reportedly staying at a hostel in Auckland. Police have released a CCTV image of Millane at Sky City – an entertainment complex.
Her family are from Wickford, near Basildon, and said they chatted to her “most days.” They described her silence as “completely out of character” – and confirmed that she planned to travel to other parts of New Zealand and then intended to go to Australia.
“Clearly this is a very stressful time for Grace’s family and the longer she remains missing the more our concern grows for her safety and wellbeing,” added Beard.
A UK foreign office spokesperson stated: “We are providing consular support to the family in the UK, and working with the New Zealand police in relation to reports of a missing British national in New Zealand.”
Several sightings have been reported, but none confirmed.