Police are appealing for help to trace the last movements of a man before he was hit by a truck on one of Tauranga's busiest expressways.

The 39-year-old was critically injured when he was struck by the logging truck about 4.40am yesterday on Takitimu Drive, above Chapel St.

The incident sparked the partial closure of Tauranga Harbour Bridge until just after 8am as police and emergency services worked to treat the man and clear the scene.

He was taken to Tauranga Hospital, then was transferred to Waikato Hospital's critical care unit, where he remained in a critical condition last night.

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Acting Senior Sergeant Trinity Milham said police were still trying to work out how the man came to be on the highway, where there was no pedestrian access and an 80km/h speed limit. The section of highway was a key arterial route for the Port of Tauranga and was used regularly by heavy port-bound vehicles.

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"Really, up until that crash has occurred, and as to what his movements are and what brought him up onto the motorway, we are no better or further ahead in our investigations," Ms Milham said.

"We are still seeking witnesses that may have seen him prior to or had a face-to-face encounter with him." There was no description of the man available yesterday but police were keen to hear from anyone who noticed a man in the Takitimu Drive, Chapel St area about that time.

The man was last seen in the CBD about midnight, Ms Milham said.

Why he was on the highway, or how he got on there was still a mystery yesterday, especially in such poor weather, she said.

Torrential rain drenched the city early yesterday.

"Obviously we are unable to speak to the key person who would have the answers."

She said she hoped the man's family might be able to provide information about his whereabouts and activity between midnight and 5am.

"We would just like to sort out the event that brought him onto this motorway," she added.

"If anybody is in a position to come forward and talk to us, we would love to hear from them."

People can contact police at the Tauranga police station on 07 577 4300 or provide anonymous information via 0800 CRIME STOPPERS.

Businesses in the area said although there was access to the highway from a Sulphur Point walkaway, they had never encountered pedestrians on that section of the overpass.