Optus is expected to launch third-generation high-speed mobile phone services in Sydney and Melbourne on Monday next week.

The carrier has called a media conference "to make a significant announcement" on Monday morning where it is expected to outline 3G services on the back of its AU$700 million network sharing alliance with Vodafone announced this time last year.

3G is a generic name for higher-speed mobile phone and wireless data card network services, that allow users to access rich media such as video content and make video calls through their handsets, as well as being able to get Internet access anywhere a mobile phone can get coverage.

Optus said in its annual results presentation today that it would supplement its existing 3G service in Canberra -- launched in April -- with offerings in Sydney and Melbourne in the first instance.

An Optus spokesperson wouldn't confirm the launch date this week, saying only "we have always maintained that we will launch before Christmas and are on track to do so".

The SingTel subsidiary will be the fourth mobile carrier to launch 3G services in Australia, following early entrant Hutchison with its '3' network, as well as Telstra which launched in early September and Vodafone which went live with its own offerings several weeks ago.

Optus' coverage is expected to mirror Vodafone's, due to the fact they share the same network. Vodafone's chief marketing officer John Casey said at launch its network covered 75 percent of Sydney, 55 percent of Melbourne, and 90 percent of Canberra.