twest I really like this answer, but I'd like to add a slight counter argument and expand on it a little. If yours or your clients website is very rarely looked up, then most ISP's won't even know who you are... at this point they'd still need to go to a root server and then refer to your server. ISP's in Aus likely us a APAC root DNS, if that has to then poll a Mexico server there will be a latency of +150ms added to that TTFB.
That said your servers aren't geo-dns aware or advertising. So if your enhance cluster has 3 dns servers, 1 in USA, one in UK and one in AUS... It's literally round robin of your NS records which will get polled by the root DNS server.
Cloudflare helps here, as they totally geo-dns there infrastructure. You can even buy geo-dns for servers from them... but at that point you'd might as well use them for all DNS needs.
I don't use CF for my DNS most the time and currently have 1 NS server on US Westcoast and one in Germany. Any ISP lookup for the USA might get sent to Germany because of the round robin effect and visa versa. It is what it is.
If you're serving all your clients in AUS alone, then having one DNS server on each end is fine, or as @twest said, just make sure they're in difference DC's. Being closer to the APAC region will help with minimising TTFB, when it's not cached with an ISP.
twest "(some respect tls, some don't gaf and use their own config)..." bloody hate this, but I get why they do it. It's almost like whats the point in a TTL?
