Pete from Australia,
Our most common roofs these days are Colorbond, a product made by BHP. They are corrugated metal that doesn't rust.
They oxidise after time and are best pressure cleaned using a decent surface cleaner.
It's the nature of the product. BHP recommend using 2% SH but that does very little, the easiest way we have found to remove it and make it new again is pressure.
I would love if somebody can recommend a product that remove heavy oxidation.