Q.Salvia Leucantha
Something is affecting my Salvia Leucanthas. They appear to be withering, then die. I live in Kwa-Zulu Natal, South Africa. 30 Kilometers from the coast.

Though they can be drought tolerant when established, they do need water when they are newly planted. You should check the soil, when the top couple of inches of soil are dry, water again.
Salvia will wilt in the hot part of the day even if well watered, so take care not to over water the plants. Over watering can cause root rot and the death of the plants.
Soil should be well draining.
https://www.gardeningknowhow.com/ornamental/flowers/salvia/growing-different-salvia-types.htm