A lot of countries do ask for contact information on a custom form, in case there's a need for the US customs to contact you about your package at the border. It's unlikely that it will be used, but it's also a formality for some shipping companies as well - particularly courier companies or if you use a particular type of postage (ie EMS is one of those, from Asian countries).
...I can't speak for specifics on Aus. but it's not uncommon for those details to be requested.