The variety of broadband service providers able to service your area will often depend on which wholesaler owns the copper line that runs to your house. This line is owned by one of the major telecommunications wholesalers, either Optus or Telstra who then rent the line to allow other broadband service providers to offer an ADSL service.
Telstra generally owns most copper phone lines connected before 1997, before a de-regulation of the telecommunications industry opened the market for Optus to begin laying their own infrastructure.
When provisioning a home phone or broadband service Optus will either rent the existing copper line from Telstra or install their own infrastructure. If the latter is the case you will need to have a home line re-connected by Telstra to apply for a broadband service with another service provider. Because Optus uses different broadband technology, you are restricted in the providers that can offer you a broadband service on Optus infrastructure.
To make it a little more confusing, if Telstra still owns your phone line but Optus is renting it and billing you for the home phone service, you will generally still be able to get an ADSL service with another provider. You would need to speak to either a Telstra or Optus to confirm who actually owns the copper line running to you house.
The majority of broadband ADSL, ADSL2+, Cable or Naked DSL providers will require you to have a home phone service on the Telstra network prior to you broadband application. This is due to agreement within the telecommunications industry about renting copper line to provide a broadband service. Most providers have a deal with Telstra allowing them to rent this line to provide a home phone or ADSL service to your home.
On Optus infrastructure you will be able to run broadband services from Optus. iPrimus may also be able to offer a service depending on the type of infrastructure at you r address. If you are on an Optus phone service, you can also apply for a mobile broadband service with any provider, as a mobile broadband service does not run through the home phone line. You can compare mobile broadband plans from iPrimus, Dodo, Vodafone, Exetel and Bigpond at Compare Broadband.