Hi Glen,
Thanks for your suggestion.

You have control over the TLDs and Whois Servers to which your WHMCS installation can connect, so can add the necessary records with immediate effect.
Add the following to resources/domains/additionalfields.php:

$additionaldomainfields[".au"] = $additionaldomainfields[".com.au"];

Then add the following to resources/domains/whois.json:

{
"extensions": ".au,.asn.au,.com.au,.edu.au,.org.au,.net.au,.id.au",
"uri": "socket://domaincheck.auda.org.au",
"available": "---Available"
},

Related documentation:
- https://docs.whmcs.com/Additional_Domain_Fields#Add_a_new_field
- https://docs.whmcs.com/WHOIS_Servers#Customising_WHOIS_Servers