Sydney's busiest bus route is the 400 from Burwood in the west to Bondi Junction via Sydney Airport, data from NSW's Opal electronic ticketing system shows.

About 16,000 journeys are taken on the 400 service – the only bus that drops passengers at the airport – on a typical week day.

The crowning of the 400 as the city's busiest route comes as the Baird government encourages commuters to use phone apps to find out how crowded a bus is before it arrives, allowing them to decide whether to board or wait for the next service.

The NextThere app is the first to provide a real-time assessment of crowding on buses, and began a live feed to users on Wednesday.