Emergency services were called to the scene near Chain Hills, between the Mosgiel on-ramp and the Fairfield off-ramp on State Highway 1, about 11.30am.
Both vehicles were travelling north.
A man, a New Zealand Post staff member, tried to resuscitate the driver of the van while waiting for emergency services to arrive.
St John spokesman Ian Henderson said the van driver died at the scene.
The left rear indicator lens on the trailer of the heavy truck was damaged in the crash.
The driver's side of the white Toyota van was badly damaged.
The northbound lane of the motorway was closed and traffic diverted yesterday as the Serious Crash Unit investigated.
The section of motorway reopened at 4.20pm.
Police asked motorists travelling in the southbound lane, who might have seen the crash, to call police.
-Dunedin police (03) 471-4800.
