Looks to be sort of fungal issue but hard to tell what just from that photo. Are these seedlings or established plants? Indoors or out? Where are you located?

Photos of an entire plant or two would help.

Are you sure it is not bugs?

Most of us are not too familiar with what may be problems in South Africa, but more information would be helpful.
What is the soil like?
How well does that soil drain?
What is the weather like? You are heading into spring and maybe summer now, right?

It kinda looks like tea scale that get on the bottom of Camelia leaves.

aphids and scale.

Jimmz22, I would go for 'scale insect'; but please do not ask me "which one?".
Here is a pretty concise treatment of the subject - albeit a bit out of date.

If that is scale the usual method of control is with horticultural oil sprays, either dormant or superior. Some sources do mention pesticides which will also harm any predators of the problem as well as the pollinators.

Here is a link that might be useful: Control of scale on plants