Apex domains cannot have CNAME records.
Question and response
Question
dig sisgroup.com.au. NS
Answer
sisgroup.com.au. 5 NS ns2.sisgroup.com.au.
sisgroup.com.au. 5 NS ns1.sisgroup.com.au.
sisgroup.com.au. 5 NS ns1.us.sisgroup.com.au.
Authority
The response has no authority records.
Additional
ns1.sisgroup.com.au. 5 A 122.202.94.94
ns1.sisgroup.com.au. 5 A 175.45.132.102
ns2.sisgroup.com.au. 5 A 203.29.240.87
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0