Does a CDN help if all my visitors are Australian?

Often yes, though less dramatically than for an international audience:

• Distance still applies within Australia - a server in Sydney is a measurable distance from Perth.
• Offloading matters more - the CDN serves images and files so your origin handles fewer requests.
• It absorbs spikes - a busy day or a bot sweep hits the CDN rather than your hosting.
• Check the edge locations - a CDN with no Australian presence can make things worse, not better.

If your host is already in Australia and your traffic is modest, fix images and caching first.