An Auckland woman believes she saw the young British tourist, who has been reported missing by her family, near the Southern Motorway in Manukau.

Grace 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 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.

"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

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.

"We tried ringing up the hostel, which is the Base Backpackers in Auckland, which were very helpful trying to track her down," he told Newstalk ZB.

"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."

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

Base Backpackers declined to comment to the Herald yesterday.