Members of the shelduck family, including Egyptian Geese, and more closely related to ducks than geese. Egyptian geese can hybridize with mallards, which true geese cannot.
I would follow the recommendations for feeding Egyptian goslings. Non-medicated chick, duck, or gamebird starter, with plenty of greens like lettuce, but not spinach. I'm taking this all directly from this thread: https://www.backyardchickens.com/threads/what-to-feed-egyptian-geese.116483/
Could they be Australian Shelduck babies? Those look very similar to Egyptian geese babies, and they are native to that area.
If they do turn out to be Egyptian geese, they should not be allowed out in the wild, because feral populations of that species spread rapidly and will out compete...