Major mobile phone companies in Oz n Nz are :

Vodafone Australia

Welcome to Vodafone Australia's home page for personal mobiles - check out special offers, links to My Vodafone set up, paying bills online and more.
www.vodafone.com.au/

Optus

Optus is an Australian leader in mobile phones, broadband & home phones, delivering cutting-edge networks & entertainment services.
www.optus.com.au/

Telstra Australia

Discover Telstra's range of fixed phone, mobile, Internet and Pay TV solutions. Buy securely online, configure your services, access Online Billing, ...
www.telstra.com.au/

Three Mobile Australia

3 Mobile, three.com.au for the latest 3G mobile phones and Cap plans for mobile internet, mobile broadband, mobile email, music and much more.
www.three.com.au/

Another good shop is

Mobile Phones | Crazy John's

The biggest range at the best prices - Mobile Phones, Caps, Plans, Prepaid, Mobile Phone Accessories and more.
www.crazyjohns.com.au/