Tehau put his Tahiti side in front scoring in the 12th minute.
But Madang equalized through captain Neil Hans to leave the score locked at 1-1 at half-time.
The match looked certain to end in a draw until the 90th when Tehau found the back of the net again to give AS Venus third place in the final Pool C standings of the OFC Champions League 2018.
Madang finished last in fourth spot with no wins.
In the match after, Auckland City FC edged past Lautoka FC 1-0 to finish first in Pool C, and secure a home quarter-final.
Pool C final standings:
1) Auckland City FC
2) Lautoka FC
3) AS Venus
4) Madang FC