Wanganui's reigning NZ champion Dan Cowper suffered a number of breakdowns and had to settle for sixth place, leaving him second overall behind Seymour.
Palmerston's Derek Smyth, a member of the Wanganui club, finished 10th after two second places in the earlier rounds and lies fourth in the national series, with Wanganui's Mike Gee-Taylor in fifth.
And Sunday saw two section wins for local drivers - Hamish Auret in class C (up to 4 cylinders, super-modified), and Grant Harrison in class E (up to 4 cylinders, super production).
The national trials series is run in seven classes and each driver's best five rounds out of the six count in the championship which next heads to Whitemans Valley, Wellington, on January 29, then to Te Puke on March 3 and ends at Thames on April 7.
Sunday class winners
Class A: Graeme Goble (Mt Egmont)
Class C: Hamish Auret (Wanganui)
Class D: Mitch Seymour (Windy City, Wellington)
Class E: Grant Harrison (Wanganui)
Class F: Graeme Kingstone (Bay of Plenty)
Sunday overall result:
1, Mitch Seymour & Mark Stockler (Windy City, class D)
2, Brian Howat & Dwayne Mead (Windy City, class D)
3, Ron Sturme & William Sturme (Eastern Bay Twin Diff Club, class D).