Naked Domain and WWW domain Cached Separately??

I have my site setup using site and home url at https://www.mydomain.com.au
I have a cname record pointing www to my domain

WP Supercache is creating separate caches for https://www.mydomian.com.au and mydomain.com.au

This is causing issues with some users being served different cached versions of the site.

Why are separate caches being created and how do I prevent it?