A member of the public found her and called emergency services about 7.30am.
The victim was in hospital last night in a serious but stable condition.
Northland police area commander Inspector Justin Rogers said she was being supported by family, police and Victim Support.
"This is a terrible crime and our thoughts are tonight with the victim."
Whangarei mayor Sheryl Mai last night said she was "gutted, devastated" to hear of the attack in an area dubbed the "jewel in the city's crown".
"Our fabulous Hatea Loop - everyone loves it. ... of the hundreds of thousands who use the Loop, it's horrid to think some predator has taken the chance to do this ghastly deed to a precious young woman - that's the horror."
Council had met yesterday with the view to fast-tracking lighting planned for the Riverside Drive section of the Hatea Loop.
Last August, a 69-year-old man was stabbed in the back in Riverside Park during an evening walk. He spent four days in hospital following the attack.
In lighting the track, the council had been contending with problems around wiring on Pohe Island, an old dump site which released methane.
"The worst part is we know [the lights] are in our work programme," the mayor said. "Who knows whether the lights would've prevented this but, regardless, we'll be working to reduce the chance of something like this ever happening again."
The mayor praised the police's quick work. Following the incident being called in, front-line police, dog patrols and CIB members were deployed.
Just before 1pm yesterday, police stopped a car which matched the description of a "vehicle of interest".
The driver was taken to Whangarei police station and the car was seized for forensic examination.
A residential Whangarei address will also undergo a forensic scene examination by police and ESR today.
The accused was interviewed by police for several hours yesterday and was in custody last night. He is due to appear in Whangarei District Court this morning. Mr Rogers said the community would be "shocked and angered" by the assault.
"We have put every resource available to us into this case from early this morning and I hope this arrest will bring some degree of comfort to the residents of Whangarei."
Police want to hear from anyone who may have seen a blue Nissan Serena people mover with tinted windows and silver trim below the door line in Whangarei, particularly around the Hatea Loop area, in the days leading up to this attack.
