It is expected the serious crash unit might need several weeks to determine what happened but New Zealand Police and Interpol have begun the process of finding and notifying the dead man's next-of-kin.
Emergency services were called to the scene at 12.13am and the motorway was temporary closed between the Princess St on-ramp and the Mt Wellington Highway off-ramp.
Any available CCTV footage from motorway cameras will also be reviewed as part of the SCU's investigation.
The young man's name is not expected to be released today.