mail.instra.net.au has no NS records.
NS records for this zone are defined at instra.net.au.
NS records for this zone are defined at instra.net.au.
Question and response
Question
dig mail.instra.net.au. NS
Answer
The response has no answer records.
Authority
instra.net.au. 600 SOA ns1.instradns.com. root.instra.com. 2014082502 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0