An Auckland woman believes she saw missing British tourist Grace Millane near the Southern Motorway in Manukau.

Millane arrived in New Zealand around two weeks ago and had been in constant contact with her family. The 22-year-old was last in contact with her family on Thursday, November 29, and concerns for her safety are mounting.

The witness, who asked not to be named, told the Herald she believed she saw her standing by the Manukau on-ramp.

"Where she was standing, it was really bizarre and she was by herself, nobody around just a girl on her own," she said.

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"I can honestly 100 per cent say I saw a young girl, absolutely so close to her description that it's not funny, and there was no way I was going to stop.

"She was in a very unusual place to stand, I would have thought a hitchhiker would have stopped on the left-hand side to get in the car, she was on the other side."

Grace Millane has not been in contact with her family in the United Kingdom for several days. Photo / Supplied
Grace Millane has not been in contact with her family in the United Kingdom for several days. Photo / Supplied

The witness said the young woman, who she believes to have been Millane, did not have her arm out with her thumb stuck up in the air like a hitchhiker.

She could not remember if the young woman had bags with her or not or whether she saw the woman standing by the road on Friday or Monday afternoon.

"I looked at the photo [of Millane] and I was quite adamant I had seen that girl," she said.

"I just saw a young girl and I thought she had like a raincoat ... she had a bigger sort of jacket, not a fashion like one."

Millane's family sparked the investigation into her disappearance yesterday after they became alarmed when she didn't respond to birthday messages on Sunday.

Her older brother Michael Millane said it was completely out of character for her not to be in contact with her family.

Millane was about six weeks into a year-long world trip. Before New Zealand she had spent four weeks travelling in South America.

During that time she had been in touch with her family daily through WhatsApp.

"We tried ringing up the hostel, which is the Base Backpackers in Auckland, which were very helpful trying to track her down," her brother said.

"She hasn't really replied to any messages between the family and it was her birthday on Sunday and she didn't respond to any messages."

The family put the call out there because they were worried for her safety and wanted "to make sure she's alright".

Her brother was not sure of her travel plans in New Zealand, but he knew she had a flexible itinerary booked to travel around most of the country.

She had paid up to stay at Base Backpackers until December 8.

While she was travelling alone, Michael Millane said she had made a lot of friends.

Her father had left England Wednesday evening (England time) and was due to arrive in New Zealand on Friday.

"It is a really hard situation for the family at the moment," Michael Millanesaid.

The family had been in "touch constantly" with New Zealand police.

Base Backpackers declined to comment to the Herald on Wednesday.