Apex domains cannot have CNAME records.
Question and response
Question
dig mx.bigpond.com. NS
Answer
mx.bigpond.com. 5 NS adns03.bigpond.com.
mx.bigpond.com. 5 NS adns02.bigpond.com.
mx.bigpond.com. 5 NS adns01.bigpond.com.
mx.bigpond.com. 5 NS adns04.bigpond.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0