Wostmann, a lecturer at the Wits School of Accountancy, chased down defending champion Nina Podnebesnova in the closing stages to stun the elite women’s field.
She crossed the line in 3:41:25 in wet, windy conditions, becoming the first local athlete to bag the ladies’ crown in 14 years.
“I feel absolutely over moon. I’m so happy,” Wostmann said.
“I did not expect this kind of outcome at all and I’m absolutely thrilled.
“I was hoping to finish at the back end of the top 10, so this is much better than I anticipated.”
Podnebesnova held on to take second place in 3:44:06, and South African debutant Tanith Maxwell was third in 3:45:20.
While five SA women grabbed gold medals, Mthandazo Qhina, who took fourth place, was the only local runner to finish among the men’s top 10.
Nkhabutlane, who was second in the annual 56km race in 2011, broke clear of Zimbabwean Collen Makaza with less than seven kilometres remaining to win in 3:10:29.
Makaza, the world 50km champion, took second position in 3:12:43, and Moeketsi Mosuhli of Lesotho ended third in 3:13:46.
Stephen Mokoka won the men’s half-marathon in 1:04:00, four seconds ahead of Elroy Gelant, and Lebogang Phalula pocketed the women’s title in 1:14:48, finishing 20 seconds clear of second-placed Mapaseka Makhanya.