When using cloudflare’s CDN through a Siteground hosting account, will it use ALL of cloudflare’s datacenters to serve the websites? Including the ones in Australia? Or is it limited to just the main ones around the world?

I just did a test through the Sydney datacenter for one of my free plan sites. So, yes, you get full access as long as the datacenter isn’t heavily loaded and prioritizing paid plans.

Cheers mate.
So your site is hosted with Siteground?

Nope. Cloudflare isn’t affected by where the site is hosted, but I’m using a US Datacenter.