The event, in which some of the world's major cities will be temporarily plunged into darkness, is hoped to spotlight the global need to reduce carbon emissions.
Organisers say the initiative, which started in Sydney last year, will be observed in 35 nations and across 370 cities, towns and councils worldwide.
It will see lights switched off in major buildings and public places for an hour from 8pm on March 29, while householders are urged to break out low-carbon emitting candles.
"We're asking for the whole hour ... and for cities around the world it is whenever that time rolls around," World Wildlife Fund communications manager Adam Harper said.
"Whenever eight o'clock pm on March 29 rolls around, for one hour, switch off the lights.
"People in the Sydney area, and in all the partner cities, should see a dramatic change in the city skyline ... as icons are plunged into darkness."
Mr Harper said an estimated 2.2 million Sydneysiders took part in the first Earth Hour last year, and it shaved 10.2 per cent off the city's energy consumption for the hour.
Cities outside the Australian capitals expected to take part this year include Christchurch, Bangkok, Seoul, Dubai, Antarctica's Casey Base, Manila, Copenhagen, Rome, Dublin, Atlanta, Chicago, San Francisco and Mexico City.
Organisers have not set a global energy reduction target this year, saying it would only confuse the Earth Hour message.
"We don't want to set a (carbon emission) tonnage target. That's just a very positive by-product of what the event is about," Mr Harper said.
"The point behind Earth Hour is ... it's more than just one hour of one day, it's about every hour of every day of our lives and how we can make small changes that can make a big difference."
Mr Harper said candlelit picnics and dinner parties would be held across Australia to mark the event, while rocker Pete Murray would perform unplugged at an Earth Hour event in Sydney.
"The image of our city in darkness and communities gathering by candlelight inspired the rest of the world," NSW Premier Morris Iemma said.
"We should be proud as a city that we have led this extraordinary movement and I congratulate Earth Hour organisers for their vision."
NSW opposition leader Barry O'Farrell said a neon sign at his electorate office had remained off since his daughter asked him to pull its plug last Earth Hour.
In Brisbane's CBD, 190 buildings and 104 neon lights are registered to switch off and go an hour without power.
Lights in Melbourne's icons including Federation Square, Eureka Towers Skydeck and the Rialto Towers will also be switched off.
The federal parliament building in Canberra will be blacked out, while the ACT's Federal Golf Club is planning night golf with players using luminous golf balls on the putting greens.
"There's an Earth Hour wedding that we have heard of. People are coming up with all sorts of activities that they can do in the dark. It's becoming a real outing," Mr Harper said.
© 2008 Australian Associated Press