Scale. Most people use neem oil because it's organic, but I would pull out the big guns and use malathion. I like malathion because it is a great product and works fast and have used it for years.

Thank you for advice I sprayed with an organic botanical oil. I may go the other route. Is it worth cutting affected leaves? if left untreated what will happen can the tree cope naturally with then? Also sorry for all the question is it possible for it to spread to my other trees?

You can use cotton balls soaked with rubbing alcohol to wipe the visible scale off then rinse the leaves with plain water, and finally apply some type of horticultural oil ("all-season" or "light/ultralight"). Scale is a PITA so it requires multiple sprays over several weeks to keep it under control. You don't have to remove the leaves but some people with older trees that have more dense branching and foliage that is infested with scale, often prune those branches out completely (but yours isn't that size from what I can see in your pics).

That said, if you can somehow control the ants (often hard when the plant is outside) that will help with keeping them from "farming" scale and/or aphids on the trees. If you see ants (or even aphids), you can spray them off with plain water through a hose or hand-held sprayer.

Thanks for the great advice!

Unfortunately in Australia we don't have rubbing alcohol instead we have Metholated Spirits. Is this safe to use to wipe leaves?