UPDATED: 3.41PM Police believe foul play was involved in the events that led to a woman lying with serious injuries in a gutter just north of Auckland.

The woman remains in a critical condition in hospital with serious head injuries, after being found on the side of a road in Dome Valley this morning.

Inspector Mark Fergus said police are keeping an open mind as to where and what caused her injuries.

He said at the moment they're focusing on identifying the woman, and as she's unable to speak to police, witnesses will be crucial in solving the case.

The woman is described as being in her 20s to 30s, Caucasian, with short dark hair, and has a distinctive tattoo of a rose with the words 'mum' and 'dad' written through it.