Keh shot rounds of 71, 70, 68 and 71 to finish 16-under-par on 280, with her third round, a 6-under-par 68, not only ensuring a large lead, but setting a women's course record on the revamped St Clair course.
St Clair is certainly a happy hunting ground for the 20-year-old Aucklander as she was a member of the team which won the women's Toro tournament at St Clair in December.
Keh can now add the South Island title to a list of recent successes which includes the national amateur title last year.
''My approaches were very good this weekend which gave me many opportunities and I putted well to make birdies,'' she said.
Keh has made rapid progress in the past two years and rates Lydia Ko as a real role model for the Auckland girls.
''I have enjoyed being on trips in New Zealand teams with her and have certainly learnt a lot,'' she said.
Julianne Alvarez (Manor Park) came in second place, with Munchin's younger sister Wenyung (Pakuranga) coming in third, a further six shots back.
Sarah Bradley (Cambridge) was the first-round leader with a 4-under-par 70 on Saturday morning with Munchin Keh and Alvarez one shot back on 71.
Wenyung Keh was handy on 72, while Celyn Khoo (Pakuranga) and In Sun Choi (Coringa) finished on 73.
But the scoring was not as good in the windy afternoon conditions, with Bradley finishing on 80 to drop out of contention.
With a 4-under 70, Munchin Keh took control, finishing 7-under-par for the day to take the halfway lead.
Keh had a five-shot buffer over Alvarez going into the final day, with Hanna Seifert (Lochiel) a further shot back.
But Keh's record-breaking third round meant that there was little excitement in the final round.
