Cyclists are travelling at an average of almost twice the signposted speed limit during peak hour on one of Sydney's busiest pedestrian thoroughfares, a Sydney Morning Herald speed camera test has revealed.

While authorities say rangers are monitoring the Pyrmont Bridge around the clock, many riders are more than doubling the 10km/h advisory speed limit for the shared pedestrian and cyclist pathway.

The Herald used a hired speed camera to measure how fast cyclists were riding across the bridge for one hour during peak hour on a Thursday morning.

Of the nearly two dozen cyclists monitored, the highest speed recorded was more than twice the limit at 23 km/h, while the overall average was slightly lower at 18.6 km/h.

Cyclists say the limit is too low and difficult to obey without losing balance while pedestrian advocates are calling for new laws to make speeding on a bike a fineable offence.