Apex domains cannot have CNAME records.
Question and response
Question
dig optus.com.au. NS
Answer
optus.com.au. 5 NS dns0.optus.net.au.
optus.com.au. 5 NS dns1.optus.net.au.
optus.com.au. 5 NS a1-84.akam.net.
optus.com.au. 5 NS a8-67.akam.net.
optus.com.au. 5 NS a4-65.akam.net.
optus.com.au. 5 NS a5-66.akam.net.
optus.com.au. 5 NS a3-64.akam.net.
optus.com.au. 5 NS a14-67.akam.net.
Authority
The response has no authority records.
Additional
dns0.optus.net.au. 5 A 198.142.231.61
dns1.optus.net.au. 5 A 198.142.231.63
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0