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According to this document, you do not need a transit visa. This is because you stay in the transit zone (Croatia is not a Schengen member) and as a South-African you do not feature on the list of countries which need a transit visa:
- Afghanistan
- Kamerun
- Γthiopien
- Kongo (dem. Republik)
- Bangladesch
- Nigeria
- Eritrea
- Pakistan
- Ghana
- Somalia
- Irak
- Sri Lanka
- Iran
- Syrien
- TΓΌrkei