Can compare your coin with this choice example:
http://www.sterlingcurrency.com.au/items/gold-half-sovereign-1915-sydney-mint-choice-unc-ms-63.

Looks a pretty damn fair copy to me, but an expert might differ :-)

Should definitely be 19mm diam though. If it is real gold, even if it is a fake, it's still worth around the $200 mark.