Sunlight alone does not explain it. Another possible factor is diet. It can be that eating a lot of carbohydrates with high glycemic index like white rice trigger slightly excessive growth of eye balls in children sufficient to trigger myopia. That also explain why moving to Australia affects this due to diet change.

But this hypothesis also has problems. For example, less refined carbohydrates apparently do not lead to this. Also it does not explain how adding more meat to refined carbohydrates affects the mechanism.