Apex domains cannot have CNAME records.
Question and response
Question
dig ext.sitehost.co.nz. NS
Answer
ext.sitehost.co.nz. 5 NS ns1.sitehost.co.nz.
ext.sitehost.co.nz. 5 NS ns2.sitehost.co.nz.
Authority
The response has no authority records.
Additional
ns1.sitehost.co.nz. 5 A 223.165.67.4
ns2.sitehost.co.nz. 5 A 120.138.23.11
ns1.sitehost.co.nz. 5 AAAA 2403:7000:4000:c00:0:0:0:4
ns2.sitehost.co.nz. 5 AAAA 2403:7000:4000:100:0:0:0:e
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0