Both sides beset by rudimentary mistakes throughout the game and the even the only try came from an error, with Seymour picking off a loose pass from John Hardie to decide the contest.
Rupeni Caucaunibuca, in his first game for the province in a decade, showed a few glimpses of his glory days and some silky running in the opening half looked to have handed his side a second try, but his hand off to Whiria Meltzer was deemed forward.
Despite ceding the best scoring opportunities, Southland dominated much of the game in terms of territory but constantly failed to apply the finishing touch, exemplified before the halftime break when a prolonged period of possession inside the Northland 22 came to nothing.
Southland's toothlessness was again on display early in the second spell when Cardiff Vaega shrugged off several tackles to get near the line, only for Marty McKenzie to shovel a pass over the sideline when the Stags had numbers down the right.
Following that brief burst of excitement, the second half settled into the same uneventful pattern which plagued the first, with Scott Eade's third penalty narrowing the deficit to one.
With 15 minutes to play Eade missed a tough shot at goal which would have given the Stags their first lead, and the first five wasn't the only one to err. Southland continued to enjoy the majority of the ball in the latter stages but were continually undone by mistakes, with Seymour's second penalty providing Northland a bit of breathing room.
Northland 13 (Seymour try; con, 2 pens)
Southland 9 (Eade 3 pens)
HT: 10-6