@schama
This the Australian regulation imposed since 2007 for new cars:
"The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h."
With the example that you've given, your dashcam could be showing you the true speed while your speedo was configured to indicate a higher speed than the actual which is still within the allowable limits set under the regulation.
My dashcam also shows a lower readings compared to my speedo but the difference is only +2-5 kph