Apex domains cannot have CNAME records.
Question and response
Question
dig adsl.xtra.co.nz. NS
Answer
adsl.xtra.co.nz. 5 NS ns-1926.awsdns-48.co.uk.
adsl.xtra.co.nz. 5 NS ns-94.awsdns-11.com.
adsl.xtra.co.nz. 5 NS ns-811.awsdns-37.net.
adsl.xtra.co.nz. 5 NS ns-1066.awsdns-05.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0