Double-decker buses are returning to Sydney, as part of a government trial to free road space across the city.

The first bus, unveiled this morning by the Transport Minister, Gladys Berejiklian, will enter service on Monday, running routes between Blacktown and the north-west suburbs.

And seven more will start in the new year, operating routes between the city, the north-west, and the northern beaches as part of the trial to run until 2014.

For a city starved of road space during morning and afternoon peak hours, double-deck buses bring their pros and cons.