Apex domains cannot have CNAME records.
Question and response
Question
dig nsw.optusnet.com.au. NS
Answer
nsw.optusnet.com.au. 2 NS dns1-slave02.syd.optusnet.com.au.
nsw.optusnet.com.au. 2 NS dns1-slave01.syd.optusnet.com.au.
nsw.optusnet.com.au. 2 NS ns1.syd.optusnet.com.au.
nsw.optusnet.com.au. 2 NS ns2.mel.optusnet.com.au.
nsw.optusnet.com.au. 2 NS dns2-slave02.mel.optusnet.com.au.
nsw.optusnet.com.au. 2 NS dns2-slave01.mel.optusnet.com.au.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0