Missing backpacker Grace Millane was last seen at a hotel with a "male companion", police have revealed.
The 22-year-old, from Essex, described by her father as "lovely" and "family-orientated", vanished in New Zealand six days ago.
Detectives in Auckland say they have spoken to the man and have identified an apartment at the Citylife Hotel as a location of interest.
Det Insp Scott Beard said police "hold grave fears for her safety".
Miss Millane's father David told a press conference in Auckland: "Grace is a lovely, outgoing, fun-loving, family-orientated daughter.
"She's usually in daily contact with either her mother, myself, her two brothers or members of the family on social media."
Miss Millane, who graduated from the University of Lincoln in September, had been travelling alone in New Zealand for two weeks, following a six-week group trip through South America.
She had booked to stay at the Base Backpackers hostel in central Auckland until 8 December. She is known to have made plans to travel after that to other parts of the country, followed by a trip to Australia.
Mr Millane said: "She... has been bombarding us with numerous photographs and messages of her adventures.
"We are all extremely upset and it's very difficult at this time to fully describe the range of emotions we are going through.
"Finally, I would like to take this opportunity to appeal to anybody who has seen, spoken to, come into contact with Grace over the last few days and come forward with any detail, no matter how small."
The family has set up a Facebook page - Help Find Grace.
The Foreign Office said it was providing support to Auckland police.
Prayers will be said for Miss Millane at Our Lady of Good Counsel Roman Catholic Church in Wickford, where she lives with her family, over the weekend.
Father John Corbyn said: "We will obviously be praying for her and her family at this anxious time.
"People will be very worried about her, particularly those who know her. Anybody is welcome to come, the church is open during the week if people want to come in and say a prayer or light a candle."