Apex domains cannot have CNAME records.
Question and response
Question
dig extmail.bigpond.com. NS
Answer
extmail.bigpond.com. 5 NS exhdnsf5.bigpond.com.
extmail.bigpond.com. 5 NS cladnsf5.bigpond.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0