Mystery surrounds circumstances that led to a Mount Maunganui woman being hit by a car on one of Western Bay of Plenty's notorious road black spots.

The 26-year-old had been walking alongside State Highway 2 near Plummers Point Rd on Thursday night when she was struck by a passing car about 9.20pm.

She was critically injured and rushed to Tauranga Hospital, then flown to Waikato for further treatment. The woman remained in critical condition.

Head of Western Bay of Plenty road policing, Senior Sergeant Ian Campion, said it was not clear why the woman was walking along the highway.

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The driver of the car stopped at the scene and has been co-operating with police but Mr Campion said he was keen to hear from others.

"Police are not looking for any other individual involved in the crash but obviously if there are witnesses who haven't yet spoken to police, we would like to talk with them."

The highway was closed for about four hours as Serious Crash Unit investigators and ambulance staff worked to clear the crash site.

"Police arrived very shortly after the crash occurred. A passing police patrol stopped and assisted at the scene," Mr Campion said.

The stretch of road had been identified as a "high risk" area set to benefit from a $73 million revamp of Bay of Plenty's and Waikato's most dangerous roads.

Last month, Transport Minister Simon Bridges announced the investment as part of a $600 million Safer Roads and Roadsides Programme aimed at improving more than 90 high-risk sites.

The safety improvements would include realignment of some corners to improve visibility, side barriers, median barriers, rumble strips, wide centre lines, road marking and improved signage in areas including SH29 over the Kaimai Range, SH2 north of Katikati and the SH2 corridor.

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Last month, two people were taken to Tauranga Hospital after a head-on collision.

In February two motorbikes crashed on the corner of SH2 at Whakamarama. In November a Tauranga man was seriously injured and had to be cut from the car he was in after it hit a ditch and catapulted through a fence at Whakamarama, and in May last year Auckland women Tevunga Vaea, 71, her daughter Louina Vaea, 45, and daughter-in-law Takua Vaea, 32, died after their minivan and a car collided on SH2 between Whakamarama and Te Puna.

In August local man Alexander Taiatini, 44, was killed when he was hit by a truck and trailer unit while walking along SH2 at Paengaroa.