I am no DNS expert, but the authoritative nameservers for .com.au and other .au
domains include

ns1.ausregistry.net. 203.18.56.41 
ns2.ausregistry.net. 203.18.56.42
ns3.ausregistry.net. 203.18.56.43
ns4.ausregistry.net. 210.8.15.253

None of these IP address resolve to a hostname except 210.8.15.253 which
resolves to DAUS139537-1.gw.connect.com.au.15.8.210.in-addr.arpa

Doesn't it seems strange that the organisation largely responsible for running
.au does not have the IP addresses of its own nameservers resolving to hostnames?

Further, 
- ns3.ausregistry.net does respond as authoritative 
- ns.iinet.net.au which is also listed as authoritative for com.au does not
agree  on serial number


Looking at .net.au, .org.au, id.au, shows similar irregularities.



CT