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The crash, on State Highway 1 near Evansdale, closed the road for more than three hours.
Emergency services were called to the scene shortly before 7.30am.
The driver of the four-wheel-drive was trapped for some time after the crash, a police spokeswoman said.
"He was ... extricated and transported to Dunedin Hospital in a serious condition."
The road reopened shortly after 11am.
A St John spokesman said one ambulance was sent to tend to the injured man. The driver of the petrol tanker was uninjured.
The petrol tanker had come to rest on the shoulder of the north-bound lane facing south, while the four-wheel-drive — which had panel damage to its driver’s side — was facing north in a ditch beside the north-bound lane.
Debris and skid marks were visible on the road between where the vehicles came to rest.
Following the crash, traffic was heavily backed up along State Highway 1.
Police inquiries into the crash are continuing.