A fourth person has been arrested in the investigation into the alleged assault of a 19-year-old woman who was found unconscious, dumped on the side of a road in Dome Valley, north of Auckland last week.

A 29-year-old woman will appear in the North Shore District Court today, charged with kidnapping.

Police said they were not ruling out the possibility of further arrests as the enquiry unfolded.

The victim remains in Auckland City Hospital in a serious but stable condition.

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Yesterday, three people appeared at the North Shore District Court, charged for their parts in the alleged kidnapping and threats to kill the mystery tattooed woman who was found bound and gagged with critical head injuries.

The trio, who have name suppression, is made up of one man, 38, and two women, aged 28 and 42.

It is understood the victim was taken to an address on Great North Rd against her will.

While she was there she was bound foot and hand with cable ties and attacked with a hammer before she was dumped on the road.

A member of the public contacted emergency services after they discovered the woman, aged in her 20s to 30s, about 7am.

Following her discovery on May 9, police used a photo of the woman's distinctive rose tattoo featuring the words 'Mum' and 'Dad' to it to appeal for help to identify her.