Apex domains cannot have CNAME records.
Question and response
Question
dig optusnet.com.au. NS
Answer
optusnet.com.au. 5 NS a24-66.akam.net.
optusnet.com.au. 5 NS a26-66.akam.net.
optusnet.com.au. 5 NS a1-70.akam.net.
optusnet.com.au. 5 NS a9-67.akam.net.
optusnet.com.au. 5 NS ns1.optusnet.com.au.
optusnet.com.au. 5 NS a11-65.akam.net.
optusnet.com.au. 5 NS a2-65.akam.net.
optusnet.com.au. 5 NS ns2.optusnet.com.au.
Authority
The response has no authority records.
Additional
ns1.optusnet.com.au. 5 A 203.2.75.2
ns2.optusnet.com.au. 5 A 203.2.75.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0