Part of the relaunch of Earth Hour, volunteers will use the candles to spell the words ''Lights out for the Reef'', reflecting a new focus for the annual World Wide Fund for Nature event.

Experienced environmental campaigner Anna Rose took the reins of Earth Hour Australia four months ago and admitted its message had been due for a refreshing.

Earth Hour would become a year round social movement called Earth Action, Ms Rose said, with a training camp for 50 budding environmental activists planned for later in the year.

''It's a different focus for Earth Hour, from just this one moment when people turn off the lights into really building the power of the Earth Hour movement and building Earth Hour supporters into a powerful constituency who can take action and exert pressure year round,'' she said.

Organisers chose to highlight a specific example of the effects of climate change, this year damage to Queensland's Great Barrier Reef.

Ms Rose said without immediate action to combat climate change, the damage to the reef would be irreversible by 2030.

''Having a focus on the reef reminds people why Earth Hour was started in the first place, the fact that if we don't act, this is what's at stake,'' she said.

Australian National University professor Lawrence Saha, an expert in social movements, said the popularity of recent March in March events in cities around Australia would likely increase turnout for other social and environmental movements.

''The propensity to engage in any form of political activity of a social movement type becomes easier the more often you do it; it becomes part of a person's behavioural repertoire,'' he said.

University of Canberra political scientist David Marsh said political activism outside the mainstream might not have an impact on public policy, which could lead to participants becoming even more detached from the political system.

But Professor Marsh said that did not mean they were not worthy of attention.

''They're telling you something about what the attitudes of citizens to mainstream politics is at present,'' he said.

Volunteers will use candles to spell out ''Lights out for the Reef'' on the lawns of Parliament House at 6.30pm on Saturday.

If it is rained out, the event will be moved to Haydon Allen Lecture Theatre at the ANU.

Earth Hour is encouraging Australians to turn off their lights for one hour at 8.30pm.