Sydney kicks off replacing 6450 streetlights with LEDs over coming months, to become the first city in Australia to be lit by LED lights.

It estimates to halve the amount of electricity required to keep the city lit at night, and cut Sydney's power bill by about $800,000 as well as save 2861 tonnes of greenhouse gas emissions a year.

The first stretch of new lights have now been installed outside the town hall on George Street, in time for Earth Hour, which begins at 8.30pm on March 31 night.

Along with London, New York and Hong Kong, Sydney has been trialling the lights for 18 months, as part of an experiment by think tank The Climate Group to test public reaction to the slightly different light qualities of LEDs.

The lights have been installed at Circular Quay, Martin Place, Kings Cross and Alexandria Park. According to The surveys show, 90 per cent of respondents supported the switch to the new lights.

There are about 22,000 streetlights in the council area, of which 13,500 are managed by power utility Ausgrid, and the rest by the council. Public lighting is responsible for about a third of the council's energy use and 30 per cent of its greenhouse emissions.

For Earth Hour, lights in most council buildings will be switched off for an hour from 8.30pm. The event started in Sydney in 2007, but has since caught on around the world. Governments in 130 countries have agreed to participate, and more than a billion people are expected to join in. Earth Hour is supported by Fairfax Media, the publisher of the Herald.