There is Kryal castle just near Ballarat VIC, it has activities and shows to participate in. Fun day out.
I also pass one occasionally in Gippsland – someone has built their home in a castle style.

http://www.aroundyou.com.au/articles/castles-to-explore-in-australia

Try this link. There are quite a few castles to explore

Paronella Park. https://www.facebook.com/paronellapark/

Knockrow near Byron Bay has the Macadamia Castle it has shops inside and a animal farm.

Castles are wonderful places to explore! Depending on where you are, a few off the top of my head are: Kryal Castle in Victoria; Sunshine Castle Bli Bli in QLD; Kings Plains Castle in New South Wales; and Gold Coast’s Gothic Castle.

There’s quite a few all over a quick Google search will send you in the right direction.

There’s a small one in Tasmania. Heading from hobart to the Tasman peninsula. It sits out in the bay, the only way to it a thin tract of land which sometimes disappears at high tide