Lights were switched off in various Australian landmarks across the country to mark the third annual Earth Hour.

There are hopes the climate change campaign that began in Australia two years ago with just over 2 million people taking part will reach global proportions this weekend, with 1 billion people expected to take part.

Sydney's famous harbour landmarks and many of its high rise buildings were plunged into darkness, with tens of thousands of residents and businesses also turning their lights off for Earth Hour.

Earth Hour originated in Sydney in 2007, with just 2.2 million Sydneysiders taking part.

In Sydney, city landmarks including the Harbour Bridge, Opera House, and Anzac Bridge powered down for an hour from 8.30pm (AEDT), when ferries at Circular Quay blew their horns to signal the start of the event.

The New South Wales Government says although the event is mainly symbolic, it has raised awareness of the need to conserve energy.