Certainly not the first time I have seen a copper nickel coins that looks to be pure copper.
Sometimes (and always depending on the environment), copper nickel coins can tone in this way. Usually the result of direct soil contact, in ground burial.
At a confirmed weight of 5.000 grams, I can assure the OP 100%, that this is a normal, standard Jeff.
Australia' 5 Cents, 10 Cents, 20 Cents and 50 Cents are made of exactly the same alloy.