Krylon is available down under and they have a wide variety of clear spray (rattle can) finishes in gloss, satin or matte. Most any of them would work, but I would stay away from the artist's fixative - it will wipe off. You should be able to find them at your local home center or at
The Artist Warehouse, although it will probably be more expensive at art stores.
Graham’s suggestion to use lacquer is not a bad one, but honestly I don’t see any reason a spray acrylic wouldn’t work. Unless you live in a humid, salt-air environment (which I am led to understand is not common in the land of Aus) then I don’t think you need worry a whole lot about additional oxidation on copper. If anything, I think it would tend to even out the patina.
I prefer acrylic finishes over lacquer because of drying time and clean up. Might not be the right thing for furniture that gets a lot of wear and tear, but works fine for models.
i live in a high humidity area in Australia. Humidity at the moment up around %80. Uptown 95%