Apex domains cannot have CNAME records.
Question and response
Question
dig bigpond.com. NS
Answer
bigpond.com. 5 NS a1-185.akam.net.
bigpond.com. 5 NS a2-65.akam.net.
bigpond.com. 5 NS adns05.bigpond.com.
bigpond.com. 5 NS a28-66.akam.net.
bigpond.com. 5 NS a3-65.akam.net.
bigpond.com. 5 NS a18-64.akam.net.
bigpond.com. 5 NS a11-66.akam.net.
bigpond.com. 5 NS adns06.bigpond.com.
Authority
The response has no authority records.
Additional
adns05.bigpond.com. 5 A 203.50.248.19
adns06.bigpond.com. 5 A 203.50.248.20
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0