Speed of light is the lower bound for network latency but actual internet latency also significantly affected by delays added by network equipment (routers/switches). Typically the bigger distance the more routers there are in between so this delay is also proportional to the distance. And sometimes there are almost saturated channels so there is a significant queuing delay for packets waiting to be sent. RTT AU <-> US will be higher than RTT for most pairs inside the US but RTT for the same distance within the US can vary more than 3x depending on exact network infrastructure on the way.