Speed cameras will be switched on at three of Melbourne's busiest intersections in the next fortnight.


Police said the fixed cameras are in 40km/h speed zones at the intersections of Flinders and Williams streets and Exhibition and Victoria streets in the CBD, and Fitzroy Street and Lakeside Drive in St Kilda.


The cameras were switched off in 2012 to give motorists time to adjust to reduced speed limits.


They will be turned on again some time in the next fortnight, police said. Signs will warn drivers of the changes.


Road Policing Superintendent Kevin Sheridan said he hoped the new cameras would discourage drivers from speeding in areas with large numbers of pedestrians and cyclists.


"We encourage everyone to be responsible on the roads and to drive within the required speed limits," said Mr Sheridan.


"The 40km/h speed signs are not advisory – they are the law and if you speed, you will get caught."