Apex domains cannot have CNAME records.
Question and response
Question
dig mailcluster.com.au. NS
Answer
mailcluster.com.au. 5 NS ns2.digitalpacific.com.au.
mailcluster.com.au. 5 NS ns3.digitalpacific.com.
mailcluster.com.au. 5 NS ns4.digitalpacific.com.
mailcluster.com.au. 5 NS ns1.digitalpacific.com.au.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0