As I recall virtually all Aussie cities are near the coast so are subject to humidity and salt air to some extent. Northern Australia is tropical so presumably has high humidity. None of these are good for carbon steel. One of the best corrosion protectors I have used is LPS 3, available in liquid or spray on aerosol form. When this dries it forms a waxy coating on the metal to be protected and claims to be a "premier rust inhibitor".
Unless pitted the blade should be able to be cleaned up and then treated with a good rust inhibitor. Just to change the blade finish I have personally sanded down and then polished some SS knife blades to give them a mirror finish. I have also blued carbon steel blades. Once cleaned your blade can probably be refinished to match the original finish whether that was a brushed finish, bead blasted or almost anything else.