DNS not propagating for subdomain/s

Trying to configure a subdomain but for some reason my DNS isn't propagating - I've created the new site, setting conf to:

# domain: new.mydomain.com.au
# public: /var/www/new/public_html/

 <virtualhost *:8180=""># Admin email, Server Name (domain name), and any aliases
  ServerAdmin webmaster@example.com
  ServerName  new.mydomain.com.au
  ServerAlias www.new.mydomain.com.au

  # Index file and Document Root (where the public files are located)
  DirectoryIndex index.html index.php
  DocumentRoot /var/www/new/public_html
  # Log file locations
  LogLevel warn
  ErrorLog  /var/www/new/log/error.log
  CustomLog /var/www/new/log/access.log combined</virtualhost> 

enabled the site, restarted apache, all seems fine.

Added the A record in apache, but I'm not seeing it propagate - existing subdomains (ie mail) resolve, but anything new I create does not. I'm not being impatient as there is an existing subdomain, months old, which does not resolve.

It's a site I've inherited with no handover so have spent a bit of time unravelling the mess (multiple copies of the production site on the server, under mismatched folder names, in strange locations etc), so am now wondering what I could have missed?

DNS should be straight forward, yeah?