Hi Robert!
We aim to target the Australian market that is why we want to set up a .au site, and yes, we are a bit pressed for time so instead of rewriting everything, we intend to just create .au pages for our main sections, but the inner pages would be from our .com site. They're both in English so we also think that there's a chance we would be considered having duplicate content if we rewrite all our .com pages to be used in .au.
We would definitely use geotargeting/language selection for our AU site to avoid any issues.
Thank you so much Robert for taking the time to answer my question! I truly appreciate it!