With the score poised at 14-11 to Thunder with 25 minutes remaining, the pressure was on for Thunder to hold off the Southern offensive until the final whistle, as at this point, only one try had been scored by the home team.
Unfortunately for Thunder, two further penalties were conceded without reply, so this took the final score to 17-14 to Southern.
This result means Thunder will be playing New Plymouth Old Boys at New Plymouth in the semifinal next week. Southern will be hosting Coastal in the other semi.
Players of the day were Gianni Pedemonte (three points), Henry Marchant (two points) and Shannon Haenga (one point).
Other scores:
Thunder Premiers 28-7 Southern, meeting Southern in their semifinal at Stratford.
Thunder Colts 37-12 Southern, meeting New Plymouth Old Boys at New Plymouth.