Webmail is a way of accessing your email using a web browser just like Hotmail or Gmail.
There are 2 main URL's you can use to access webmail. If one isn't working for you just try the other:
yourdomain.net.au/webmail or webmail.yourdomain.net.au
You can also use your server's URL, for example: https://vmsh37.ha-node.net/webmail