Welcome to the Bunnings Workshop community @hai2alex. It's wonderful to have you join us, and many thanks for your question about painting retaining walls.
To successfully coat a retaining wall with exterior paint, you'd need to ensure no moisture penetrates through the wall from whatever is being retained. This means that the rear side of the wall should have a waterproofing membrane on it. Otherwise, the moisture buildup in the wall will eventually push the paint off the surface of the wall.
If the wall has been waterproofed, you are free to use any exterior paint, and no primer is required for a clean masonry surface.