South African Shelduck

This is a great photo but I don't think it's an Ashy-headed Goose. The bill seems too large and the speculum isn't right. I believe that this is male South African Shelduck (Tadorna cana), which is equally uncommon in captivity. Also, Gray-headed Geese have orange legs and white flanks with black barring; both of those characteristics are missing from this bird which is why I believe it's a shelduck.

Thanks, you are spot on @Summer Tanager

This is a male hybrid South African shelduck or Cape Shelduck (Tadorna cana), not a pure specimen ... too much grey on the flanks for a pure specimen
compare with these (male in the background , female in the front)
Cape shelduck, Tadorna cana, Magdeburg Zoo, Saxony-Anhalt, Germany, 3-1-18